[Koha] some staff users cannot access the OPAC

Hi. We have got a strange problem with some of our staff users: They can access the intranet but for some reason they cannot access the OPAC. I have already tried several things - changing the password in the administration form, changing the password through the "Forgot password" process in the OPAC for example - but they still cannot log in the OPAC. Does anybody have an idea why that might be? If there is nothing that can be done, I will delete and recreate these users and hope that it works this time. I am just curious about the reason, because for most staff users it is working without a problem. Thank you very much Isabel Pineda