Guess I'm on the other side of the coin. I don't think timing is the problem. It's kuzushi, tsukuri. At contact, kuzushi. Currently, I like to think of kuzushi as destroying structure. Most people explain it as taking balance. But, you can take balance and still have an uke that's dangerous.
And after contact, you can't lose kuzushi when you move. Because then uke regains structure. So while you're moving to a position of advantage, or tsukuri, uke has structure and will negate your movement.
No, I don't think timing is the problem, especially if you follow kuzushi and tsukuri.
Another point might be slack or tension. In moving, one must take out all the slack and tension in one's own body. If not, then movements are slower and jerky, which affects one's ability to create kuzushi and also to keep kuzushi. Lagging the back leg is a symptom of slack/tension in one's body. It's called full body movement for a reason.
all IMO anyway,
Mark