I just finished a build using Anderson parts with Spikes lower parts kit - Took it to the range - shoots wonderfully - However, P-Mags, Tapco, and Troy polymer magazines will not lock into the magazine well - empty, fully loaded or loaded with 29 rds will not lock in.

Aluminum magazines by Red Jacket, Blackhawk and USGI work with no problems - empty, fully loaded and with 29 rds they go right in and lock in positively with a click.

I tried dremeling the magazine catch recess on the Tapco and one PMag - still not a lock up

Suggestions on how to fix it?

The first thing I would suggest you look at is how "tight" you have the magazine catch screwed in. The way I set the position of the Magazine catch is to push the release button as deep into the receiver as it will go and then spin the magazine catch as far as possible without it catching on or marring the Bolt Release or any high spots on the lower.