Tree Peony Blooming from the root stock?

We purchased a 'Renkaku' Tree Peony 4 seasons ago (Zone 5a - suburban Chicago, 16 miles straight west of downtown) in bloom in May 2007.
All of the directions we find online say that the graft was to have been covered by at least 1" of soil at planting time. e.g.,
http://www.degroot-inc.com/product_info.php?cPath=21_32&products_id=174
It bloomed again in 2008, but no blooms in 2009.
In 2010 it had several dark red blooms appear near the plant's base but nothing else.
The garden center isn't open yet for this Spring so we can't consult with them: we are holding our breath while we wait out the next 2 months to see what will happen this season. At least it was a very mild Winter and there are swollen growth buds near the tips of every branch.
But by now, the plants' 5th season, we were expecting it to be covered with huge white fragrant blooms and somehow I don't think that's going to happen.
Does anyone have any ideas?
TIA - the Rips

Early May '12 update: we have at least a dozen huge white blooms across the top in this bloom this Spring.
March's HOT hot weather didn't help, as we'd also had to cover the plant several nights in April when temps dipped below freezing, so we didn't lose the buds. So far, the off-shoots haven't bloomed.
I think I'll try just chopping straight down through their base now and see if that will eliminate these shoots coming from below the graft.