I see 4 issues highlighted here, 2 playstyle issues, and 2 gameplay issues. I will deal with each group separately.
Caveat: There is a lot of additional information which would help this answer, a lot of the below i relying on you knowing more than I do.
- Not accepting plot hooks
- Not accepting rewards
The above indicate that there is a disconnect between how you are running the campaign, and the expectations of the players. Maybe you are wanting a tangled web of quests and they are just looking to get to the end of the dungeon and kill the boss.
Whatever the cause of the disconnect there are 2 potential, different, solutions:
- Work out why, and roleplay around it
This is assuming that the players are stuck into playing their characters counter to your style, rather than simply not liking it as players.
You need to work out what is motivating the characters to act in this manner, and work around it.
Example 1: If they are not accepting side quests because the main quest is more urgent then you can add in a natural delay to the main quest (I recently blocked a road for example and the party knew they had x days before it re-opened an no other path around it).
Example 2: The party might not be accepting rewards because the Paladin feels it is wrong so some NPC follows the party until the Paladin is busy and then says "I know (s)he didn't want to accept these rewards, but please take them, I know you will put them to better use than I". If the whole party feels wrong maybe an entire community comes together to craft a special set of plate armor which is absolutely useless unless given to the party "We appreciate you are our protectors, this will help you defend us from greater dangers in the future, please take it because we know providing this assistance to you is the best use of our resource".
Unfortunately without more information on WHY the party are acting this way it is difficult to come up with better solutions, which brings us onto;
- Talk to your players
Admit your mistake, tell them your problems and struggles and ask them what they are looking for and why they think the situation has developed.
You need to go in with an open mind for options ranging from the players realising how they are playing counter to your intent and immediately getting back on path, to them telling you they hate how you are running things and asking you to change your style to match their expectations.
I can't really advise any further until you know how this conversation goes, but I think this is your best option.
- Split up and need to get back together
The above is a consequence of the playstlye issues, and there is little point in trying to solve them until you work out how to handle the playstyle issues. Even if you manage to fix the current issues if there are large playstyle or expectation differences more gaps will appear and you will constantly be on the defensive.
So, once you are certain that everything is sorted and you simply need to a short term fix to get everyone back on track you have a few options:
- Treasure falls, everyone is rich
This one can work in a lot of ways. It doesn't have to fall from the sky but anything from finding an unguarded dragons hoard to having a rich relative die and pass on an inheritance. Just find some way of putting money or items immediately into the groups hands.
FYI on the above, unguarded Dragon's hoards are plot hooks, not just mechanisms to enrich the party. Now they have an angry dragon after them!
- All the people the party has helped come together to reward the party
This is a slightly more roleplay focused version of the above, the good people who have tried to reward the party all started talking about how the party really should be rewarded and they get together with an offer the party cannot refuse. Maybe they hold a celebration in their honor and present them with x, y and z which they need to get back to the right equipment level.
- Adjust encounters for the current wealth level, and increase rewards for the next few levels
They are poor now, so drop the xp threshold of the next few encounters, but each enemy they drop is 50% richer than they should be and the party gets a more organic method of acquiring treasure.
- Party gets back together
If you have talked to the party I am fairly sure they will find their own reasons to get back together, but otherwise work out why the party is split, what is motivating each group and drop both objectives (Slyly) in the same place. Group A reaches an ancient ruin and goes inside. Group B finds a trapdoor under the inn. Maybe you need magic portals, maybe geography is more convenient, but somewhere inside the party meet up and their objectives align once again.
In a few sessions time the party will hardly remember how mysteriously convenient it all was. Also in the same vein as a Dragon hoard, if you needed portals now you may have made an enemy of the wizard who created them and he is looking for you!
Work out WHY this is happening before you work out how to fix it.