Does it lock back without a magazine, pulling
slide open and manually lifting the slide lock?

If slide won't manually lock open, the slide lock
itself if faulty, or angle of notch on slide is wrong
(unlikely).

Take slide off and insert an empty mag. Does
follower on magazine touch the slide lock tab,
and lift it?

If the mag follower fails to lift the
slide lock, either the mag follower is mis-molded
or not rising high enough in mag tube to reach
the tab, or the tab on slide lock is missing/short,
or angled wrong (bent upwards too far).

Function of slide lock spring is easy to check.
Remove slide, manually push slide lock up--
it should snap back down (unlocked position)
when you release it.