it can be a number of things. clogged oil line/fittings, improperly drilled/tapped head for vtec, vtec lock mechanism inside the rockers are f&$*% up, insufficient oil pressure (use a pressure gauge to measure), oil too thick, vtec too weak to notice, not enough vtec juice, bad ecu, bad wiring, bad luck, blah blah...you get the point



... but IMO i think it is related to your oil lines/fittings or how the head was tapped.