pH the runoff; if its out of whack then flush it. More than likely have what's referred to as a "lock-out". It happens; happened to me. If you're desperate you can foliage feed them for a little (if not flowering) but you need to fix the root of the problem first.
Got this info from another site
Step 1)
Leach the plants roots and growing medium using a professional leaching agent to thoroughly leach away metals, calcium, sodium, chlorides, sulphates and many other compounds, which can build up in the growing media.
Step 2)
Feed with 1/4 strength high quality complete plant food mix along with a high quality vitamin B-1 product such as Superthrive (1 drop per gallon).
Step 3)
Spray a professional stay green formula on the leaves. After 24hrs, spray the leaves with a quality vitamin B-1 product.
Feed at 25% of recommended fertilizer dosage until first signs of growth.
If it is only an N problem, rarely is, then it should respond to your nutes
