here's what i do when it happens on customer's car:
1. check pressure @ back of block
2. check pressure @ front of head
3. jump solenoid to see if vtec will activiate (easy to do on dyno since you can pull while someone jumps it)...run wires into car and jump it while you drive if you have to.
4. check pan for dents (will cavitate and lose enough pressure/vol to prevent vtec like has been stated)
5. what pan/pickup combination are you using? there is a difference in the depth between ls and gsr pickups and pans. too deep a pickup and too shallow a pan and it's like having a dented pan.
6. check/clean solenoid screens (you've already done this).
7. has the head been taken apart before? if so there may be some pins/parts missing preventing vtec locking in.
8. trying vtec @ wot or part throttle? what about temps? speed sensor working? chipped ecu or virgin? if part throttle, too cold, no speed logging in ecu, or not enough load and chipped ecu w/ vtec window then vtec won't engage either.