Hi Andysh
We too have the Omnistore 1200 and haven't had any problems tensioning the poles. From memory there's a little black buckle-thingy that you use to manually tension once you've extended the poles so they're touching the pads on the inside of the canopy bag (at the top of the caravan sidewall). My job tends to be holding the canopy out while my husband does the tensioning but it's always worked for us.
Silly question - you're definitely putting the correct pole against the caravan (not the vertical one)? But the vertical one has 'feet' so it would be difficult to mix them up.
We love ours. We also bought the tent to fit underneath but are contemplating leaving that at home for our 4-week trip to France this summer as the canopy is, we think, all we need 🙂
Good luck with it!