Signer says they signed, but Adobe Sign doesn't show it

I have a couple of documents that I've sent out for signatures from multiple people (about 25 in total, split between three documents.)  For most, it worked just fine.  But for three of them, it hasn't.  For all three, it shows that they viewed the document, and they say that they signed (including getting the "you're all set!" message after clicking sign) but I never get a notice that they signed and they show up as "waiting for signature."   I had one of them try again, and it went through, but the other two have tried twice now... nothing.  What could be causing this?  Do I have any options other than just having them repeately try to sign?