Hi, 

Any number of leaf chewing insects could be doing this, and also snails and slugs, a caterpillar etc. Try going out at night with a flashlight to see if you can catch the culprit. If it is snails and slugs there are products easily available that can help manage them and aren't harmful to other beneficial insects. 

Otherwise you can also try some diatomaceous earth around the base of the plant or dusted on the leaves a bit to help deter whatever is eating them. If it rains you will need to reapply. At least this could help them leaf out again so they can bloom. You can also hand pick off any critters and drop them into a bucket of soapy water if you can find them. 

Leaf Feeding Beetles

Slugs and Snails

Let us know if you have further questions. 

Emily

I’m pretty sure this damage is not due to slugs - maybe some holes from slugs but not this much skeletonized.   
I can’t find any evidence of insects except some smooth black excretement.   

That sounds like it could possibly be a caterpillar of some sort. Most caterpillars that would eat these plants would probably be beneficial so consider it that you are feeding something native in the ecosystem. 

Emily