This is something people have mentioned as well. If you have a transformer like this one: https://www.homedepot.com/p/IQ-America-Multi-Voltage-Wired-Doorbell-Transformer-245806/206990043?cm_mmc=Shopping|THD|G|0|G-VF-PLA-D27E-Electrical|&gclid=CjwKCAiAvf3RBRBBEiwAH5XYqOy_97JsLT6Nmdzg6m0ffCByePvd_PuWhFre8WADRHT7PCmvhDVGHxoCGIcQAvD_BwE&gclsrc=aw.ds&dclid=CPGk5JaQpNgCFY3HwAodW8sHTA
Then one has different voltages depending on where you hook it up.
Or it could have been a crappy one that was never putting out a true 16V AC.
This is why putting a meter on your power wire at the doorbell switch and then testing it at the transformer will tell you what you need to know. You should be getting the same voltage at the transformer as you do where the doorbell switch was. If you do not there is something else happening/wrong.
I would test then if it is simply not enough voltage I would replace the transformer. If it is a multi output transformer I would move the wire to the appropriate voltage required.