Doorbell dings, dongs, but doesn't ding-dong

Question

I'm replacing a doorbell chime unit. The old one worked, but the new one does not. If I connect the wire to the terminal labeled "front", it will ding. If I connect the wire to the terminal labeled "rear", it dongs. The latter makes sense as that should be so a rear or side doorbell has a different sound. But why doesn't it ding-dong when connected to "front"? The hammer moves freely up and down (no wire in the way blocking it).

The transformer measures only 11 volts. That seems low, should I replace it? If not, what else could be the problem?

Old:

New:

Answer 1

It was the transformer. I replaced it and the doorbell now works as expected. Upon closer inspection the old transformer was only a 10V model where I needed a 16V one.