I have 28 wired IP cameras and 1 WIFI camera. The only one I have issues with is the one WIFI. And I have a SOLID Unifi AP HD Pro backing up the WIFI one. Still on the search for the perfect ip POE 1080 DoorBell.
lower the bitrate or frame rate on the wifi camera. Try removing any particularly old devices or ones with the weakest signal from your network temporarily.
I have a similar setup with 33 cameras, two of which are Wifi and the rest are wired. I'm also using Unifi AP's with strong signal and close proximity to the Wifi cameras. But they still are flaky despite all of this. They work "ok" but not great and will often go out on me during the day at least 2 or 3 times, but they do come back on their own (no intervention from me). I just think they are inherently less reliable due to the wireless signal being their life-line to the system. I'm not an expert, but in my experience, you are always going to run the risk of these drop-outs unless you use hard-wired connections for your cameras. But just like you (I'm guessing here), the two cameras that you have are WIFI for a reason. In my case there was just no way of getting a cable to them....and believe me....I am very stubborn about getting cables where I want them!