What altitude? Are you going at full cruise speed at lower altitude?
There is an issue where, if you are below, say 20,000 feet, going full speed, the elevator trim runs out of room and can no longer trim. If this is the case, change the Auto Throttle mode to Manual and reduce speed. Keep an eye on the “Stab Trim” in the lower left of the MFD. If it’s right at the top, it can’t go further.
The particular issue I describe has been fixed in an update - but I’m in the SU12 beta testing (the fix might only be in the beta version, I’m not sure).