Troubleshooting form will help...

How long has it been in the same soil/container? Do you have a digital ph meter to measure the run-off ph? Milwaukee ph600 is about $20 and worth every penny.

What I'm getting at is that the ph buffers in the soil ran out (usually takes 6-8 weeks, give or take), causing ph to drop from the 6's to the natural level of peat, which is less than 5, locking out essential nutrients.