A number of things cause this, A. I bet his form when barbell benching sucks balls, he probally has found a way with the bells where he can use his strengths with the barbell his weak points wont let him cheat. Also I have to have way more info to tell you why, 1. where does he miss, does the bar just flatten him to the bench, does he miss at lock out, does he have good speed of the chest and then just stall??
2. what kind of bar training is he doing??
3. d-bells only work on one specific weak point off the chest, so he probally does not have the tris to front delts to move the weight.
4. I bet he doesnt have the stabilizer muscles built in a way that provides a big bench. when benching d-bells you do not have to set up like you do to push big weights with a bar. you do not have to sit your elbows into your lats, or have a big enough upper back and traps to push into allowing the bar to move away from you.
LET ME KNOW MORE ABOUT THESE ISSUES THEN YOU CAN BEGIN TO BRING UP HIS WEAK POINTS AN HE WILL BE BENCHING 300 IN NOT TIME…big Martin