I bought 20 Pampas grasses to use as a border about 4 years ago. I can't get any plumes. They are planted in almost full sun, grow like crazy, look very healthy, cut them down in early spring, and add bone meal 4 times to fertilize them. How come they won't produce plumes?
Since you've provided plenty of phosphorus in the bone meal, the culprit is probably too much nitrogen in the soil. This inhibits plume production as well as flowering in many plants. The high nitrogen levels could come from any fertilizer or compost you've applied to your lawn nearby, or even from a neighbor's lawn fertilizer applications.
The age of the plants could also contribute. In ideal conditions they would produce plumes after about 3 years, but perhaps your climate is a bit too cool to be ideal for them. Hopefully you will see plumes next year.