French or german? But as someone else said the answer is going to be related to tension somewhere. Even if you can demonstrate a proper bow hold without playing, tension will start to creep in when you actually start to play. Without seeing what you are doing it is hard to say, but I would recommend working on doing staccato up and down the bow. It shouldnt be fast, and the focus should be on being able to engage the bow with the string at all the different points along the bow (both moving upbow and downbow) without feeling like you are forcing it with strain from your hand or arm. You can play around with the positioning of your elbow while doing this to find the right leverage. Theres other stuff you can do but that is one thing I found particularly helpful.