When I started there was a senior student who was big, tough, strong and brutal. No-one liked training with him. I eventually just avoided him (like many others) and if I would have been a little more mature would have asked him to lighten it up a bit as Nathan suggests.
In the end he left and never returned, and I think I wasted a valuable opportunity. I had the chance to challenge myself, and I shied away from it, choosing instead to be safe and protect myself by avoiding him.
Maybe your Sensei sees you doing the same as I did: avoiding that which you need to help you grow. Technique might have nothing to do with it.
Give it a couple of months, and embrace the challenge. Then, if appropriate (and you survive
, ask Sensei what he had in mind with his request.
Who knows, maybe he thought you were the only one who could challenge them.