How to troubleshoot HVAC for room that is uncomfortably warm in both hot *and* cold weather?
With the info you have provided we really can’t help you, there are mini splits out there you can install for ~1500$  I would want see how your system is set up because your hvac guy sounds like he knows less than my neighbor and I helped him put in a mini split. Unless you can provide some specifics I vote to close.