Model is messed in Modelview?!

[Amnesiaplayer]

I made a model, a bridge like thing, for amnesia. it was supposed to be a test object, but since i worked like 2/3 days, i am going to use it as my second bridge.
i made it with like, 16 Objects (cylinder x6 and some planks- BOX scaled)
i did everything i had to do, but if i open my .dae file on modelviewer, everything was mixed up!!! like, 4 cylindeR_pyramid is standing where it is supposed, to be standing, but the plansk are GONE, the cylinders are in East china (really far away).. i heard that you need to combine you're items, i did, but it only worked for the cylinder_pyramids, if i want to combine the other is says : // Error: Invalid selection : polyUnite needs at least 2 polygonal objects.