Here are some possible solutions and reasons for vtec not to crack.

No Check Engine Light:
Dowel pin on cam cap 3 not in

Oil Pressure to Low( if LSV, check Oil line and its a good thing to have a Oil pressure gauge)

Not Enough Oil (check the oil level and make sure its inbetween the two dots or more)

Fuel Pressure/Clog (sometimes a clogged fuel filter causes it to bog up at vtec rpms)

Spark Plugs (check the wires and see if its fully down and ur spark plugs are working correctly)

Wiring (check the vtec wires and see if it is making a full connection)

Vtec Solinoid Screen Clogged (clean it out with brake or carb cleaner)

Vtec Solinoid not working (take a wire and hook it up to the vtec solinoid switch and touch it to the possitive part of the battery. if u hear it pop, it still works, if it doesnt, then u have a messed up solinoid))

Vtec RPM

Check Engine Light:
Evap Canister Leak (Check all the hoses on the black canister under the hood and check if the gas cap is on tightly)

Oil Pressure Switch Not plugged in
Vtec Soilinoid Switch not Plugged in

Other things to Check - Valve Lash, Alternator and its connection (sometimes a bad alternator will cause it not to crack) Distriberator. Try out different ECUs. Timing. If you have a cat, it can be a clog in ur cat.

Feel Free to add other experiences with no vtec, these are just some i have come across.