Depends on the type of cat litter also. I use premium clumping (or rather my cat does, before anyone gets an image in their head that can't be erased) which has a very small particle size and goes straight up the vacuum without any issue. Some non-clumping litters in particular are much coarser in texture, and will get snowploughed by the vacuum.
With the Dyson, it may be worth experimenting a little with the Min-Max slider. Set it however you prefer on carpet, but on hard floors, I'd be inclined to try opening it fully in order to raise the front seal. Hopefully this will provide enough clearance to allow larger dirt to be drawn in through the front of the head. What you don't want is for the cleaner to seal itself down onto the floor; if it's hard to push with the brush roll turned off, this is likely what's happening.
Bear in mind that a lot of uprights claim to be OK on hard floors, but this doesn't necessarily mean they're the best tool for the job in practice. I've yet to use one I'm 100% happy with for this task, and I'm not even sure if it really exists. Nine times out of ten, a combi head or dedicated hard floor tool will be easier to work with and do a far better job.