I'm using Adobe Pro DC to create and read the form. The recipient does need to create a digital signature by clicking on the signature box.
I've sent several copies of different signature document and have had the same problem; however, the last few attempts have started to work. I still can't see the signature box in the email preview, which I use to be able to do but now when I save it to file and open it the signature box is appearing again, which hasn't been the case for the past couple of weeks. I had tried to walk through this with a client last week, saving it to my hard drive, and it didn't work then.
Let's hope it keeps working, I'll have to try it on a non-Adobe Pro machine to be sure.