This seems to be a result of a new jet-engine tuning for pb12 of XP11. The engines idle too low, and can not accelerate from this low RPM. You can adjust that in planemaker:
Open "Plane Maker.exe" in the main X-Plane folder.
Click File - then Open then load the IXEG 737-300
Click "Standard" then "Engine Specs"
find the "lo idle fuel adjustment" and increase the value until the idle RPM is about 22%N1. (I don´t have this installed on my laptop, but the current value is probably 0.6 and you may want to increase it to 1.1 or so...)
Hope this helps, Jan