GarageBand monitor doesn't work

sorry for long title. so, i bought a Griffin jack for my electric guitar that i can plug into guitar, line out to speaker, line out to ipad or iphone and use iShred Live. that works fine. but then i tried it with GarageBand on my iphone, and the first time I tried it the monitor setting worked. ever since then, on neither my ipad nor iphone will the monitor setting work. I can turn it on, yes. No sound. I can make sure my settings are not on Mono, I can turn off crosstalk, I've pushed every button and tried every setting, I've reinstalled GB on iPad, no luck. if I can't hear what I'm playing it is hard for me to work with GB. are there any other fixes out there?

to be clear, the input is through the 1/8" headphone jack, not the 30-pin. this worked once before, but not since. very frustrating that there is no other information on this