If it's the same issue I had, get them to open the MobiControl client on the device - this will force the connection.

Second create a file sync rule that transfers nothing but runs a script.

All the script has to have in it is:

foregroundmode enable

Set it to run when device connects.  This forces MobiControl to always be active.  It will be permanently be listed as running in Notifications.

on a couple of devices I have tried mobicontrol doesn't open, it looks like its loading and then closes and says mobi control has stopped working

Please clarify the following:

- Approximately how many devices are there without problem?  Are they of the same brand/model as the 80 or so problematic ones?

- What are the brands/models of the problematic phones?

- What Android firmware version are they running on?

- What are the version and build numbers of your MobiControl server?   What about those for the device agent(s)?

- Are your device agents Android+, Generic Android or Android Enterprise agent?

there is roughly 600 phones that are fine,

there are all samsung j5 2017

firmware version 7.0 

server is 133.3.0.3454

device agent 13.3.3.1055

android + 

I originally suspected that your device agent is a bit too old to handle the low-power modes and other new features of the Android 7.0 firmware running on your devices.   However, if ALL the 680 or so devices are all Samsung Galaxy J5 (2017) and ALL are running Android 7.0 with the same v13.3.0 agent,  I have to rule out possibility that issue(s) in the 80 problematic devices is due to major incompatibility of the device agent with your Android 7.0 hardware. Remaining potential problems can be related to

- low-power modes

- low memory

- conflicting policies from MobiControl

etc.

 In any case, the agent version you're using  is a bit old and does not support the new Bitdefender antivirus protection engine.  So after fixing the current problem, you might need to consider upgrading both the server and device agent properly by qualified personnel.

If your problematic  devices are really out of contact for a very very long time,  it is very likely that you at least need to recall them.  Hopefully, a solution that doesn't need device factory reset can be found.  As your earlier post seemed to indicate that you have at least one problematic device on hand,  it will thus be easier to debug. 

Are your device using MobiControl kiosk mode/lockdown-menu? 

Can you go to devices' Settings and change any parameter there? 

Can you boot your phone into safe mode or force the device into ADB debug mode?