As others have said, it sounds like an extractor or ejector issue. My money is on the extractor, had the same exact issue with my Diamondback DB380 (which was complete junk). They replaced the ejector and it did the same thing. I told them from the beginning I was 99% sure it was the extractor and they didn't listen to a word I said.
As for the EC9s I'm sure Ruger will correct the issue, I know that sucks since you just sent it to them and had to wait on it but if you send it in a second time I'm willing to bet they take good care of you.
I actually just got my EC9s back from Prescott a few weeks ago. I had mine for 3 years and all of a sudden it started having issues. Turns out the slide lock had a tiny bit of rust on it where it meets the frame. I'm going to be honest, while I have a bit of a soft spot for the EC9s I'm ready to get away from it. The issues I was having made the decision easier but I don't like the manual safety or the mag disconnect. Sometime in the next few months it's going to get traded towards something better.
I love Ruger don't get me wrong but it seems like their revolvers and their newer semi autos are worlds apart. I know the old P series and the SRs have a great reputation like their revolvers but the newer semi autos just aren't the tough indestructible guns Ruger built its reputation on. That reputation is why I went with the EC9s in the first place. I'll be keeping my LCP since it can be carried when anything bigger can't be concealed but I'm looking at other options for that role too. I'm not trying to bash Ruger, I just got a Speed Six to be my new ccw once I'm done cleaning it up, I just feel that revolvers are more their strong suit.
Honestly, I never used to be a gun snob but I'm kind of starting to be, especially when it comes to semi autos. I can't lie, my Taurus G3C has been absolutely dependable but it's pretty much designed like a Glock so that could be why. Otherwise all of my lower end semi autos have malfunctioned in one way or another.
The DB380 was complete junk, to be fair I knew it's reputation but I thought after 12 or 13 years they'd have the bugs worked out. Sadly that wasn't the case so we won't count that one.
My EC9s was 100% dependable, not one issue for 3 years up until a few months ago. My LCP did have one stovepipe, however it was badly fouled at the end of a range session and it happened with target ammo. I switched to my carry ammo to make sure and it ran 100%. It hasn't done it since then but I'm keeping an eye on the situation and cleaning it weekly to be safe. Even my Glock 42 didn't like certain brands of ammo but was dependable for the most part.
My revolvers on the other hand have been great except for a few ammo related issues. A couple 22lr duds and a few issues when I first started reloading. With factory centerfire ammo and my current reloads (now that I know what I'm doing) I've had no issues. Even my cheap Rock Island 206 and EAA Windicator always went bang.
I guess what I'm trying to say is even if you have to save up a while or buy used its worth it to go for the higher end stuff. And if you absolutely must go for a cheaper gun then go for a revolver. Just make sure you check the lock up and timing but I've been much more successful with cheap revolvers than I have been with cheap semi autos. I just wish I would have realized that sooner, it would've saved me alot of money instead of replacing the junk ones. Within the next year I plan on trading and replacing all of my lower end guns. It's just not worth the risk of having a malfunction when you need it most.