If you hold the mag up and it feeds fine, it's the interface between the bb catch n the mag and the feed tube of the hopup unit. The tube isn't pushing the latch far enough to release the bbs. The first one may go...since when you put the mag in the bbs will spurt out...but subsequent shots are dry because there's no spring pressure behind them.
You can trim the catch..."KA mags not feeding" was a popular search term. It won't hurt any feeding/use of those mags in other guns.
You could also try other hopup units...but it'll be hit and miss.
You could look at your mag catch....if it has a lot of up and down play or if the edge. That engages the mags is not as high as it could be, then trying a different one might work. There's usually very little adjustment there though.
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