Edit found answers:
do not detail strip your firearm!!!!! as this may void your warranty. i have had this issue on two of my HK's. more than likely, the detent plate is mishapen or a little rough-edged. dis-assemble the gun as if you were doing a normal cleaning. you should have 5 pieces in front of you, six if you include the mag. now, on your frame, look at the safety, and look above that where it should say 2+1 or something like that. that piece your looking at with those numbers on it is your detent plate. now look in the center of that piece, where the black peg or dot runs inside as you work the safety. on the interior part of the detent where it looks like one or two consecutive little hills, that's where you have an issue. go from fire to safe, the first "hill" that your "peg" tries to "climb" is where you have a rough spot or it's completely mis-shaped. i had this issue first on my uspc 40cal, and then on my uspf 45. call hk customer service and tell them the issue and they should send you a free detent plate so you can have installed. or, if you do not have a qualified armorer, send it back to them and have hk change out the detent plate. this should cost you nothing because it is a quality control issue. i have made hk customer sevice aware of the situation and they said they would pass it up to whoever is in charge of QC..... before you send it back, make sure you clean and lube the gun real well, to make sure it just wasn't getting stuck because you hadn't cleaned properly. i personally put about two drops of oil on the detent plate and work it in to get the oil between the frame and the detent plate. once you rule out improper cleaning, then you'll know it's the detent plate. good luck!!!!
When this happened to mine I was told to use grease. I didn't have any so I tried oil, the stuff I was told would not work. It did help a little but it still had a few spots that felt rough/gritty and it would get stuck every couple of tries. When I finally applied the grease(after cleaning out the oil) I slowly jiggled the decock lever to work it in. After about five minutes of messing with it the decock started working like new. I can no longer tell which USPc it happened to. What caused this entire problem was extreme cleaning. I swear I sterilized the entire pistol. Depending on how deeply it was cleaned might be the deciding factor if the oil will fix it or if it needs grease. I would try the oil and see if that works since you already have it. If it doesn't then it is grease time. One thing is for sure, it isn't a bad part issue because it worked fine until you cleaned it. From the pics posted the pin appears to be in the correct position. Which leaves the only possible problem a lubrication issue. Hope you get it fixed soon, I was pretty bummed when mine did this.