Look on the inside of the right grip panel and see if the mag follower button is rubbing against the grip groove near the top. It wouldn't take much delay of the button pushing up on the hold open for the action to return to battery. Also check the mag for any sign of friction of the follower and/or the follower button in the final upper part of the channel it rides in. The hold open itself may be dirty or have old grease on it that delays it's rise. There isn't much else to check for.
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