take the slide off the frame, and take a look at the trigger bar where the arm extends up to contact the safety plunger. on that arm, where it bends inwards, there's going to be rubbing from it contacting the slide.
my g29 does this, as does all the new g30/29's i've looked at in gunshops that have allowed me to take the slides off. generally it's not and issue, it usually wears to a point and then stops. in rare occasions, with the SF frames, the rubbing causes failure to return to battery issues like yours. usually utting a few hundred rounds through it will resolve the issue, and you've already done that. it's been an issue for some time that glock's never resolved.
the number on the top of the cruciform on your trigger bar will end in "-1-". this is the updated trigger bar glock came out with that was supposed to eliminate the rubbing. well it doesn't. my g29 has this "updated" trigger bar and still rubs. i never had any reliability issues at all (mines a non-SF g29) but i called glock anyway and they sent me a new trigger bar (same -1-), and a new safety plunger. haven't taken it to the range yet with these new parts.
try calling and getting these parts to resolve the issue before sending it in. you'll have to get a cert glock armorer to do it, as they won't send you these parts unless you are one.
also, i did in the meantime put in a smoothface trigger that still rubbed, although not as much. this is the non "updated" trigger bar without the "-1-" at the end of the number. they sent me the wrong safety plunger that i swapped out at my buddy's gunshop. glock's run by a bunch of morons these days.