As said, check the diameter of the seat post to the seat tube. It should be tight with no side wobble but able to move up and down easily or with some friction, preferable with some friction, very light friction or a little more. If so, then you need to purchase Tacx, a carbon paste that will help prevent the seat post from sliding, or visit a bike store for some. You need very very very little on your seat post as when you push the seat post down, the Tacx will be forced upwards and out.