States not changing for some users

We have an AS 1 course that is published to our Kallidus LMS, it works fine for majority of users. however we have a small handful experiencing an error.

Basically, we have a contents slide that has a range of buttons, all but one are disabled as the initial state. You click on the first and it does 3 things:

1. Changes state of existing button to "Visited"

2. Changes state of a subsequent button to "Normal"

3. Jumps to a subject slide

For these couple of users on returning to the contents slide the state of the subsequent button is back to disabled even though briefly you see it change to Normal before going to 3. above.

What we have ruled out is:

1. Its not device specific, they have used it on other devices with same issue

2. Its not user specific (as far as we can make out) as they have tried to also access over our generic Citrix network with same issue

3. Its not our LMS as all other users are fine with the same course

any ideas if some software may be running in the background of the course player or something that may be causing these few users to experience the course differently to others?

any help or ideas would be great