Bad lifter

Use tons of assembly lube, 30 weight break-in oil, and make sure you don't crank that engine over too much before it fires. If it cranks a lot before firing (i.e. timing off/180* out), then redo the assembly lube again (that's what Comp recommends). Hopefully you're not running a super heavy spring either, that could be an issue. Keep the rpm above 1800-2000 and even blip throttle to 3000rpms a few times to seat the rings. 20 mins - half hour seems to always work well. Avoid idling as much as possible until you get some miles on it, that can be hard on the cam. Just my 2cents.