Variables not staying in Advanced Actions

I've been having some issues this week with variables in Captivate 9 when used in Advanced Actions.

When I use the Assign function in an action to add a value to a variable then save my action, often the variable disappears after saving. This is happening about a third of time I use them. The same variables are used in other actions without the same issue.

Does anyone know what might be causing this? The only fix I've found is to create new variables and use those in place of the ones that don't save. The problem with that is I then have to update all the actions where the variables are used.

Thanks

Alistar