Feats, Items, special feat items, and initial character build.
At level 13, she should have the following as a striker:
- Primary stat of (18 at level 1, +3 from levlling up) = +5
- Enhancement bonus of +3 from item
- +6 half level
- +2 wand/rod expertise
- +1 superior implement training (accurate or specialized implement)
- +2 combat advantage from hidden sniper + shadow walk
3 feats can add +5 to hit quite trivally.
This means, that (save if you're using old monsters from MM1 versus fort, you should be attacking versus level+12 or 25. The "average monster" attack at this level is Level+3, meaning there's a serious problem if you're under it.
Your base to-hit as given is 55%, which is completely average/adequate as attacks go for even level monsters. Depending on party composition this is unfortunate, and if you're attacking fort all the time, MM1 and 2 mobs have unusually high fortitudes. Personally, I've found myself being resentful of accuracy when it drops below 65-70%.
If you apply the bonuses above, that 55% turns into a 80%, which is far more acceptable.
For general advice, look at the charop handbook. Without more specific build details, I cannot offer specific build advice (which is better done in chat).