About two months ago, I purchased a RollerFeeder (rollerfeeder.com) as it is promoted to keep larger birds like grackles from the seed. The feeder has lived up to this claim, but cardinals thus far will not use it, and other songbirds are making fewer trips to the feeder. Does anyone else have experience with this product, and if so, do you know of anything I can do to encourage cardinals to feed?