Hey Kerry,
Please give us insight by submitting some fights 
They are layed out here: How to Properly Report Bugs!
What might help is in coop, to:
- let everyone leave for a few seconds, rejoin
- switch up who connects first
- if one person is experiencing issues more severe, let them try with an VPN active
- Change what server you set your Realm in
Please get back to me!
Silas