They have to make the saves when they enter and if they stay inside the area.
If someone enter the area of the entanglement spell, it rolls both Reflex and Fortitude twice, if it fails the first it gets the entangled condition (with the possibility to end it on its turn) and if it fails the second it is sickened until it remains in the area of the spell and 1d4 rounds more if it leaves it.
Let’s say it succeeds both saves and it moves but stays in the area of the spell. As it ends its turn in the area it makes a Fortitude save to avoid becomeing sickened (“Any creature that enters the area or ends its turn there must succeed at a Fortitude save or be sickened”). You have your turn you do something and end your turn. As the creature is still in area of your spell and it is free, it has to roll another FortitudeReflex save to avoid becomeing entangled (“those that remain in the area must save again at the end of your turn.”).
Continue this as long as the creature is in the area of the spell. (So it is not save when enter and save once when still inside, but save when enter and save as many times as they remain inside the area or the spell rans out)