I have found that when uke has the option of taking a fall or not, whether or not it is a roll or breakfall it means one of two things. Either the nage's techniques is wanting and or uke's attack wasn't very sincere.
To an extent... Nage's technique may be fine, but he may have consciously or unconsciously chosen to give uke a choice, perhaps because of the difficulty of some of the ukemi options.
My teacher warns people not to judge their own technique by uke's failure to take a particular fall. Maybe uke did something dumb. Maybe uke disconnected just a little early because they were anticipating the fall. If nage ends up stable, balanced, and in a safe or (better) advantageous position, they did fine.