question about turbo 350 trans

Hello, I rebuilt a late 70's early 80's 350 turbo for a guy and he told me it is not wanting to go into high gear unless you are kind of hard
on the gas then it doesn't want to stay in high gear all I did was put in new clutches and band he said he would do the valve body . he is trying to blame me for the problem I have rebuilt a few of them and I told him that it could be 3 things that I could think of ,detent cable not adjusted right or sticking, modulator valve or vacuum line or the valve body has a problem is there something I could of missed in the clutches or any other ideas? thanks
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