Problem with fields saving

I have created a form and distributed it to clients. The majority are able to successfully complete and submit the . In a few cases the data does not save in two fields and as these are mandatory fields the user cannot then submit the form. Both of the fields are drop down boxes. Other drop down boxes and other field types work fine. Any advice would be appreciated as I cannot figure out what is happening here. If I email the user a fresh copy of the form it works fine for them