At Timetastic we see occasional requests from customers experiencing problems with Google Calendar, usually Google calendar stops updating changes to their Timetastic calendar.
Unfortunately resolution of this issue tends to be a waiting game 🙁
By way of some background, the way an ical feed works is that it is your calendar application (Google in this case) which periodically requests an update of the information from Timetastic i.e. Timetastic does not push the data to you calendar, it's your calendar that requests it from Timetastic.
Although Google updates the calendar automatically from the Timetastic feed the timescales for this are somewhat unclear and there is no manual way to force the update.
Some observers have suggested Google updates every few hours, but Google themselves quote it may take up to 12 hours, although in in exceptional cases we have seen it take as long as 48 hours.
I wish we could provide something more concrete on this subject, but without any further specifics from Google or guidance from them on how to force a refresh, it's just a matter of time.
Note: Since writing this article in May 2015 Google have altered their timeframe for update from 8 hours to 12 hours - which seems to indicate it's an ongoing problem and they are not taking steps to improve the situation.
And finally, sometimes you can take comfort that you're not alone - here's a thread on Google's forums that's been running for over 2 years of people complaining about feeds not updating Google Calendar not updating published calendar added by URL