ok if you took a running stock sr20ve and installed n1's and it dropped a valve either the cams have a issue(outer limits of specs) or you installed the cams wrong. Its very easy for timing to change when installing cams. I always ziptie the chain to the sprockets and block the chain so it can't move. Since the same cams were involved in 3 valves dropping I would start there. Did you drop the same valve every time? What install tool did you use to put the keepers in?