"My Magazines aren't feeding in my KP-15"
"My bolt won't lock back with my magazines in my KP-15"

The KP-15 is designed to work with USGI aluminum magazines and Gen 3 P-Mags.  If you are experiencing problems with other magazines try USGI or Gen 3 P-Mags first.  Gen 3 P-Mags are notable for their over insertion tab located at the rear.  Gen 2 and earlier P-Mags have different tolerancing including shorter followers that may or may not work with the the KP-15 reliably.

If you are using Gen 3 P-Mags or USGI and your bolt still doesn't lock back, confirm operating system is tuned properly based on the ejection pattern.