Whatever the detent is, that holds the Solo's slide-lock down until the magazine's follower hits it, has become too weak.
It may be that there's still sticky anti-rust grease in the detent's hole or notch.
It may be that the detent's spring fell out, so it isn't functioning.
Something like that, anyway.
As each round is fired, its recoil is moving the slide-lock lever upward, and it catches the slide.
Look for too much "wiggle" or looseness in the movement of the slide-lock lever. Trace the reason for that, and correct it.