Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect that you have an unreliable component in your WiFi system. It could be the WiFi in the printer.
If your WiFi router is configured to be 'open' i.e. without any encryption whatever on the signal, then that in itself is an invitation to disruption and hacking, and I advise you to try even WEP if not WPA (much better, doesn't slow things down much if at all.)
If you are restarting your router or having it Off overnight or the like, then the WiFi won't necessarily work after you turn it On, unless the WiFi printer has a static IP address (i.e. a fixed IP Address, one that does not change when the router restarts).
After that, eliminate sources of local radio noise interference like small electric motors (or large ones perhaps), or cordless phones, or microwave ovens, etc.
And after that, if restarting the printer (or giving it a rest, turned Off, for 10+ minutes) seems to cure the problem, then you probably have a defective printer, in the WiFi part anyway.
But if restarting the router is what gets it working again, then you have an unreliable router... esp. likely if it is a 'cheap' brand-name router.