First of all, the Marching Mountains are full of traps. See, whenever I step forward I get hit with a fireball.
Obvious solution? Have Imoen find them all. Problem? She can't.
It's not like she has low points in trap-finding...she just can't. I'll sit there all day and she still can't bring them up on the screen. Is there a glitch? Or are they just un-findable and I'll have to brave the fire?

It's not full of traps they just don't dissapear after they trigger. I doesn't matter wheter you stand there all day or just a few seconds it only depends on your find traps %. Have you tried find traps spell?

You should have brought along the Ring of Danger Sense just for the occassions when Imoen does have to find those really hard traps. She then finds them no problem.

Don't forget Potion of Perception! It gives +20% to all thieving skills for 6 hours. :)

with boots of speed trap doesnt even detect you went over it so it doesnt trigger. You gen give them to one char. and pass and then to second and so on and on. Its boring and slow but it does the trick ;)

Presumably this only works if you already know where the traps are? I assume you're doing this for the first time, so any of the options above would work. Personally my favourite is the Find Traps spell; alternatively you might send a mage ahead with the Spell Trap of the Staff of the Magi.

The time you spend to find traps has effect on the chance of the successful trap detecting!
But it is true that if she could not find a trap after cca. 35 seconds, then she will not detect there anything further (so there is no trap OR there is a trap, which is not detectable with the specific % of detect traps)

While Imoen has always been able to detect all the traps in my game, one thing comes to mind. Try casting a restoration spell on her and see if that helps.

If not, try the other methods listed here.