Thank you for your post into the forum and sorry that you have encountered a problem.
I suspect that this issue will be difficult to diagnose via this forum, therefore a support call has been created for you, the call reference number is F0062015.
Can you please send an e-mail to firstname.lastname@example.org
with the above call reference number in the subject field of the e-mail. Please attach a copy of the Base Monitor Log Files and We Server Log files for me to analyse.
Can you please confirm the number of SQL Servers you are monitoring?
Also if the SQL Monitor Base Monitor is installed on a Virtual Machine?
I am aware of some situations where SQL Monitor may cease monitoring in the manner you have described, hopefully reviewing the log files and the answers to my questions will confirm what I suspect.