How warm is the pack getting at end of charge? Nimh should be noticeably warm, if not you are likely experiencing a "false peak". Restart the charger.

Also check each of the six cells for temp, if one is much hotter than the others it could be a bad cell.

Some chargers have a lockout feature to prevent premature shut off in the first few minutes. What charger do you have?

Welcome to the forum.