Brakes seizing up and not stopping

It is interesting that this is only happening sometimes. This tells me that we may have an issue with the master cylinder or possibly just low brake fluid. When you say the brakes don't stop the vehicle, does the brake pedal go to the floor? If so, we need to suspect the master cylinder is the issue. Here are a couple guides that will help with figuring out what the possible causes are:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Let's first and foremost start with checking the brake fluid level and if it is low, we need to find out where it is going. We don't want to just top it off because that is not solving anything as this system is a sealed system and should not lose any fluid.

Let me know what you find and we can go from there. Thanks