Hi Sam,
I think you may need to review all your 'interfaces' between the parts, and possibly simplify your model for analysis.
AFAIK, assembly constraints have no effect in Simulate - all they do is define the (initial) positions of the components.
If you look at Model Setup within Simulate, I think you'll find that the default interface is 'bonded'. This means that any two surfaces which are coincident are considered to be effectively welded together, as continuous solid material. I suspect that all your bolts and pins are functioning like this, so various connections that you think are pivots may actually be rigid. For the rollers, I suspect that they're not quite touching the column and so Simulate has not 'bonded' the two solids. If you want it to detect when two solids come into contact, you need to create a 'contact' interface.
I haven't tried analysing a structure / mechanism like this in Simulate, so others may have better thoughts on how best to do it; but I would consider suppressing all the bolts and pins and replacing them with combinations of rigid links and spring elements (to transmit shear loads but permit rotation), as well as setting the default interface to 'free' and creating specific interfaces (from the Refine tab, IIRC) wherever they're really needed.
HTH!