Tom,
Your description of the top rounds failing to strip and feed properly indicate your magazines need some attention:
1. dirt, rust, or debris inside magazine preventing follower movement. Strip magazine by using a nail or drift punch to hold down spring after pressing down follower. Remove follower, spring, clean interior and magazine parts with oily rag. Reassemble.
2. magazine spring incorrectly assembled (top coil should push UP on front of follower). Disassemble as in (1), and inspect.
3. weakened magazine spring from leaving fully loaded and compressed. This is the least likely source of the problem. Good quality magazines can stay fully loaded for YEARS without giving problems.