Is the stock painted? If so, it is probably causing fit issues.
You can verify where the problem issue in couple different ways. Depending on the colour of the stock, you can for example briefly colour the edge lines where the receiver would fit into with a sharpie and press the receiver in. The scrub marks on the sharpie line will show what is rubbing.
Similarly, you can take a light colour grease pencil(china marker) and colour the areas of the receiver that would contact the inside of the stock. Press receiver in and take out to fit contact.
Then if required, you can take extra fine grit sandpaper and polish those surfaces going slow. Check for fit frequently.
Also, keep in mind that if the stock is painted, the trigger group contact points may also be affected.