Are you tying the trailers up tightly, or loosely? Do the stems seem to bend at sharp angles--rather than a smooth curve--when you tie them up? What are you feeding it with? If the plant is healthy, it should ideally send vigorous shoots up the trellis, and hang on itself with the leaf stems winding about the bamboo.

Clematis needs a bit more than bamboo canes to climb up but once it gets going a single montana will cover a very large shed in no time at all. You could try attaching some netting to the shed if you don't have proper trellis. All we have is wires across the shed about a foot apart attached to the shed with eye-ties. The Clematis gallops all over the place and needs serious pruning each year.

Agree with Moongrower, put a support in place so it can scramble up on its own. Clematis mesh will do, trellis lathes are to thick to be much use, but eyelets and wires will do the job too.

If you need to tie it use foam covered ties not plastic ones as the will cut through the stem and stop it growing.

In my experience, young, tender clematis stems react really badly to being fiddled about with and often die back after you try to tie them in.