Issue with Iterative Solver

I am new to structural analysis, and was running a simple case to check that I understand how reference temperature and birth/death feature work. I have two bricks connected to each other over a bonded area. The two bricks have different reference temperatures for thermal expansion. I run 4 different time steps at different temperatures to check that the thermal stresses make sense and I understand the birth and death feature. It all appears to be making sense, and it solves without errors using the direct solver.

For my real case, the geometry is more complicated and the mesh is large enough that I do not have enough RAM to run the simulation. I have switched to iterative solver, since iterative solver uses a lot less RAM. However, when I switch this model  (and my real model) to iterative, I get this error:

 *** ERROR ***                           CP =      17.312   TIME= 18:18:22
 The preconditioned conjugate gradient solver failed with an error code 
 of 1.  Please check for an insufficiently constrained model.           
 Switching to the sparse direct model may allow this nonlinear analysis 
 to continue beyond this point

As far as I can tell, the model is properly constrained. I have also noticed that when i turn "large deflections" on in the solver controls, it works for this case, but not for my real model. 

Any ideas on how to fix this or why iterative solver isnt converging?

Thanks in advance,

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