It's probably not an assembly issue. If it was, you'd have a very hard time seating the mags in the well or getting them to lock in.
Come to think of it, I can't really imagine the mags being the cause of eject failures, unless the tops of the mags are seriously intruding into the ejection port. I just tried mine, and the ProMag is way more clear of the port than my old beater RamLine mag, and it doesn't cause any feed or eject issues either.
How many rounds have you put through the mag? Although I had no issues with my 9/22 mags, I understand that some have had to run a few loads through them to get them broken in.
The most likely issue is probably ammo-related, IMHO. What have you been shooting through it? If it's anything low-powered or low-velocity, that might be part of the problem.
I'm just guessing here, but I'd look at ammo issues first, before taking the kit apart. As a good marker for assembly issues, if the 4 small screws (that go down through the receiver cover into the Ruger sight rail holes) lined up & screwed in easily, you got it together absolutely right. If you were off even 1/10 mm, they would be a bear to get in.