Most of the time, this malfunction comes from the iPhone which is too "charged": the phone's processor load is too high.
To correct this problem, do the following.
1) First, check the calendar settings on the iPhone by following the explanations on this page.
2) We will now check the CPU load of your iPhone:
install the free Battery Memory System Status Monitor application on your iPhone
launch this application
you get an iPhone charge analysis screen
if the CPU load ("Usage") is 90% or more, you need to clean your iPhone to free up enough CPU to update the Simple CRM Calendar correctly.
To do so, carry out the manipulations described on this page
3) Then go to Settings -> Mail, Contacts, Calendar -> Calendar (at the bottom of the window) and set the general settings this way :
4) Delete the calendars you don't use:
go to Settings -> Mail, Contacts, Calendar
Click on each account to delete and touch "Delete".
5) To finish: turn off the iPhone, wait 45 seconds and restart.
If this still does not work, go to Setting -> iCloud -> and disable Calendar synchronization with iCloud.
Then delete all calendar accounts, turn off the iPhone, wait 45 seconds, restart and add the Simple CRM calendar again.