The first thing to check is on the Monitoring Criteria panel, what are you specifying for
Specify target systems in SYSPLEX: (default is local system only)
System ===> > > >
In the documentation for Creating a Monitor Definition it states for that section:
Specify Target Systems for Parallel Sysplex
CA MAT allows you to view an entire sysplex as a single system. To monitor a job that is running on a specific system (or one of several systems), type the name of the system in the System field. You can specify up to four system names. Each name can specify a single system or a generic name by using a wildcard indicator; for example, PR*.
Depending on what you specify for System, CA MAT monitors a job as follows:
No target systems specified (blank): CA MAT monitors jobs that are running in the local system only. This option is known as a local request. The default setting is No target systems specified.
One target system: CA MAT monitors a job only if it is running in the specified system. Any jobs with the same name that are executing on a different system are ignored. This option is known as a target request.
Multiple target systems: CA MAT monitors the first job to execute on any of the specified systems. CA MAT only monitors the first job to start, and any other matching jobs are ignored.
All systems: (*) CA MAT monitors jobs that are running on all systems in the sysplex. CA MAT only monitors the first job to start, and any other matching jobs are ignored.
Data is sent back to the local system for recording because allocation of a data set on the target system may be precluded by shared DASD restrictions, security, or SMS DASD policies. If the local system is not available, recording takes place on the target system.
Make sure you are valuing this section with an "*", for "All systems"...
If you are, than another possibility is that the MAT server is not running on the LPAR that the user is specifying on the Monitoring Criteria panel.
If you specify a LPAR and the MAT server in not active than such measurement gets deferred and invocation happens when the given MAT server starts up on that LPAR.