It's totally normal for pups to slip up. Housebreaking takes a lot of time and patience. My pups are 6 months (we brought them home at 8 weeks) and they still have the occasional accident if one of us isn't paying attention. My male pup, Kaos, always goes to the back door when he has to go, but if we're not paying attention he comes back after a few minutes and goes in the house. In fact (I found this hysterical even though I probably shouldn't lol) just last night, my husband and I were laying on the sofa with our pack. I had our oldest Ciara literally laying on me and I saw Kaos go to the door. I told Kyle that Kaos needed to go out since he was encumbered by an additional 75 lbs on top of him. He proceeded to tell me that Kaos was only sniffing something (he was being L-A-Z-Y!) so I said, "Ok but when he pee's in the house, you're cleaning it up!" Needless to say after a few minutes, Kaos proceeds to come back up the steps, walks into the living room right in front of us, lifts his legs and pees all over Kyle's boots!!!!!!! I know I should have corrected him but all I could do was laugh hysterically and say "I told you so!" Clearly, Kyle did not find it as amusing as I did but he had to laugh too and admit I was right!
Nala, our little girl, is better about holding it but not as good at telling us when she has to go. Fortunately, Kaos has to go more often so Nala doesn't really have accidents because she goes out at the same time as Kaos (sometimes together, sometimes apart one after the other). Plus I have the added benefit of the pups having big sister Ciara to help teach them. It's so true that dogs learn more from each other than us sometimes as Ciara has taught them a lot; some good, some not so good :-).
Hang in there, it will happen just be patient, expect some accidents, and be as consistent as possible.