-like someone already said, check the oil level.
-what kind of oil are you using? synthetic or non-synthetic? i've read somewhere that sometimes synthetic doesn't allow the vtec to function properly.(not sure about this statement cuz i've never tested that theory, it's something i read).
-take the vtec solenoid apart and check the screen to make sure it's not clogged. this is something that happens and most people can't figure out what the problem is so they replace the entire solenoid.
-what motor are you running(D16Z6, B16A3, B18C1, B16A, H22A, etc...)?
-what car(year and model)?
-what ecu(P28, P30, P72, etc...)?