Adding JPEG of signature seems to lock down documents for some?

I created a PDF with fillable fields that get passed back and forth between employees as several people need to date and sign the same form. The fields for the date are fillable so we just click on it and start typing. We all have our initials saved as .jpegs on our desktops so we use the fill and sign function to select our initials and place it on the form in the "initial" field (this field is NOT fillable as we're just inserting a picture). I've included an picture of the section I'm talking about:

The problem is that one or two people sometimes have issues when they're the last to sign and the form is "locked." In the example I included, the last guy who signed wasn't able to enter any text for the date and he said it was like that as soon as he opened the form. He's using Adobe Reader so he can't add text like people who have the Standard version. I'm stumped as to why this issue only happens to one or two people, they say they are doing the same steps every time and we haven't changed anything with the form as it's a template saved on a shared drive so everyone can grab a copy. Thoughts? Comments? Questions? Suggestions? TIA!