Taking a look at your Pihole logs is going to be helpful. Also knowing what kind of device is running the Pihole software may also help.
I had Pihole running on a raspberry pi 3 years ago, and I had pretty consistent issues. I've run it on other hardware since without a problem.
It could be an issue with the SD card, if you're using a raspberry pi. I've also read that the log file can grow large enough to cause issues with your Pihole instance.
So there are a number of possibilities.