Service Level Management Change Not Working Correctly

Up until last week, our Service Level Management for our UK incident response levels was Monday - Friday, 9am - 5pm.

Due to a change in support hours, we needed to change the Friday to 9am - 4pm. I made this change in the SLM calendar, however incidents seem to still be running to 5PM.

We have multiple SLM calendars, for different countries etc, but I am wondering if the default calendar would be overriding anything at all? Even through all UK response levels point to the UK calendar etc.

Any help would be appreciated, as incidents are currently breaching when no one is in the office to attend the support queues.