A common issue with these types of motors is the brushes get stuck and only one is engaging on the armature. Check to make sure the spring and brush move freely and are in good shape
There may also be a thermal switch inside the windings. It should reset, but not always and a nasty trick on cheap tools makes them just a unreplaceable fuse that means the tool is trash. I doubt Hitachi would do that.
Time to take to apart on the bench and use a meter to troubleshoot.