Calenders not saving data

Hi, new to this so bear with me We have MAC as main machine then MacBook as secondary with approx. 10 iPad all using iCal for scheduling our Engineers. All's been well since we started in 2010 but last week started having problems as data wouldn't save when we were inputting, had to re-do it 2 or 3 times kept getting a --Updating warning message

Via support we were advised to upgrade to Yosemite as we were quite old version of OS but problem still occurring.

A question is - is there a limit that iCal (now Calendar since the upgrade) can store?

We use it constantly with a diary page for each engineer each day and haven't done anything to go back in time and delete old entries - should we be doing so?

Regards

kevin
(Newbie)

How much storage space is left on the ipads? If you use them as much as you say, 4 years of entries can use a lot of space.
Then again, you don't say exactly where these errors are occurring, on the ipads or on the Mac.