340 issue after camshaft change

This sounds quite easy and makes the most sense. I did use the EO/ IC method and that’s where I think things may have gone south. Pushrods are not too long nor did the valves touch the pistons. It’s not that hot of a cam. Hydraulic lifters we’re not pumped up or filled per Comp Cams instructions. Valves have been adjusted per RustyRatRod method and appear to be fine now. I’ve built a number of engines over my time and this has been a new one for me. Still not sure why the adjustment allowed for the valves to be open just enough to not build compression before. Engine is producing compression and will plan to fire it in the next couple of weeks when I can get back to it. Thanks to those who provided good information.
What was the length of the pushrods you used? Are you using 273 type adjustable rockers?