Hi,

It might be just that it keeps you awake.
You could maybe try to lower brain usage that your mantra takes.

By doing something like shortening mantra (Yet while still knowing what this mantra means to you?), repeating it with bigger delays.
Or try relaxation techniques before attempt to enclosen your body to sleep, so there will be lower chance for inability to fall asleep.
Maybe if your doing WBTB with mantra then shortren it