It may sound trivial, but make sure the TV is working as an extended monitor with your system. If it is set to mirror the main monitor, the system may not be capable of using different resolutions. This can throw the large screen way off.
If the system is not using the TV correctly before you run OpenLP, then OpenLP cannot make it any better.
I set our large screen to a lower resolution than it is capable so that our computer could work with it. Also, our system would not handle 4K with all the other stuff running.
Fred