Welcome to Android Central! To be clear, you have an HTC One VX, correct? Because that's the forum you posted in, but your thread title only says HTC One.
It would help to know what wireless provider you use. Are you sure they provide good 4G coverage where you are? Just because they advertise 4G coverage doesn't mean that everyone can get it consistently. Compare with other people you know who have the same service. There are websites like Rootmetrics.com and Sensorly.com that will show you what reception is like for your area, but they mostly report reception for the big wireless companies, not necessarily for smaller or regional ones.
Is reception poor everywhere, or are you mostly inside a building, especially underground? That can block signal as well.
If you know that 4G reception is good in your area, then it certainly could be your phone. You might have to try a factory data reset, which would wipe the phone clean, making you start from scratch. You'd want to make sure all of your data is backed up beforehand. It's pretty easy to set your phone back up the way you had it before, but it does take a little time, since all of your apps need to reinstall. Here are instructions for a factory reset:
Perform a Master Reset with the HTC One VX (PM36100)- AT&T Wireless Support