Kirby creating dirt under carpet?

Question

My Kirby seems to work too well. Since we got our vacuum we have
been getting lumps in our carpet, mostly in the high traffic
areas. We pulled part of the carpet up too see what the problem
was and found that the bumps were piles of dirt! It seems the
Kirby is trying to **** up dirt from the subfloor, but the dirt can't
go through the carpet pad so it just forms piles. Does any one with a
Kirby have a similar problem? Is it our vacuum or the carpet, or
just a dirty subfloor? Is there anything we can do about it!