Which part of the hand is it? Can you describe the trouble a little more? Is it painful, numb, tingling, blisters, etc.

Do you wear rings while riding? If I wear my wedding band on rides >15 miles I start having tingling and weakness on the outside of that hand. Taking off the ring fixes everything. I also have trouble with gloves that have too much padding. When I have more padding, I put more weight on my hands and eventually my hands will go numb and I start having other posture-related aches and pains. With less or no padding, I'm more conscious of how my weight is distributed and maintain better posture on the bike. I also vote for trying different gloves and paying attention to using your core muscles to hold yourself up.