Looks to me some sort of a parameter that the service requires is missing, given you are saying it does not record for some of the requests.
We also need to determine the following:
1. How is the service being invoked, is it by OOTB Pega generated treatments like emails or you are calling it on your own by some other means ?
2. Did we figure out what's the difference between the ones (in terms of payload data), for the ones that is working or the ones that are failing ? that might give more clues on why some or failing.
There are also tools in the Connect services rule for IMPR-V2 to "trace" the incoming requests and see where it is failing.
Navigate to the Service HTTP rule for MKT-->RH-->IMPRv2 and click Actions-->Trace and then send the request to see the tracer entries coming in for better debug.
You could also directly run the entry point activity named "IMPRServiceHandlerV2" with the right set of arguments and debug/trace the activity while running it.
Hope that helps, if you need much more detailed information on this please create a Pega support case and work with the engineers to get more help.