do you have an error log for the problem?
it may be that some of those portal records are being edited and thus the server cannot update them because they are locked.
From an 'Under The Hood' session at a recent DevCon, we were enlightened to the fact that FileMaker Server makes no effort to update those records showing in portals on other client's screens. Records in list views are sent updates if visible records are modified, but the overhead of monitoring all related records means that the decision was made not to try to maintain updates to all these related tables.
As far as I know, your only option is to add an 'Update' button to the layout, which performs a 'Refresh Window' with 'Refresh Cache Joins' selected.
If it has been working previously, then the load on the server must have increased recently, so now it has decided it hasn't got the capacity to to keep this data up to date.
Best wishes - Alan Stirling - London, UK.
Alan is correct. Unstored calculations and related records are not typically refreshed without a window refresh. Also, if a user has a record open, cache refreshes will not happen. So, what you're describing could be a result of users just sitting on records without committing them. Or, alternatively, just sitting without causing a window refresh (i.e., by changing records or some other action).
I'm sure that's correct.