Firstly, apologies for the delay in responding here and any inconvenience this issue may have caused.
I would recommend to try reinstalling the agent/update the agent.
Try to removed and reinstall the VM agent and see if that helps.
Please check this agent not ready troubleshooting guide and see if that helps.
If the agent is not ready state, it is usually due to some corruption of the agent which can be resolved by manually reinstalling the agent.
Other reason could be issue with the image that is being used for deploying this VM. in this case support team can look into further.
As you have mentioned that this is a repeated issue in your environment, this needs deeper investigation on why the agent status is going to not ready state.
Support team will be able to check and help on this. I would recommend you to open a azure support case.