the only time my pampas flowered was after i buried my deceased cat beside it! either it was the tear jerking ceremony and prayers or do i need blood and bone meal fertiliser?
The major reason for failure to flower … and they can flower quite late in a wet year … is lack of light due to shading. ANother reason is too heavy soil that encourages leafy growth and poor flowering.
If it is not shaded, try some potash next spring, about 100 grams scattered over the root zone should do the trick.