Solver Settings
In many situations, the default in COMSOL Multiphysics when having several physics interfaces is to generate a solver sequence with a segregated solver. When several structural mechanics interfaces are present, it can happen that the degrees of freedom are placed in different segregated steps by the default solver generation. It is, however, not possible to solve a model where the structural mechanics degrees of freedom are placed in different segregated groups, so in this case you must modify the solver settings.
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If the model only consists of structural mechanics interfaces, the best option is usually to replace the segregated solver with a fully coupled solver.
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If the model contains contact conditions where the augmented Lagrangian method is used, there are two versions of the algorithm: one based on a special segregated solver, and one based on a fully coupled solver. In the first case, a segregated solver must be used. Place all structural mechanics degrees of freedom except the contact variables in one segregated step. The contact variables should remain in the lumped step, which should appear directly after the step with the other mechanical degrees of freedom.
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If there are other types of physics interfaces than structural mechanics being solved in the same study, then the segregated solver should usually be kept. Make sure that all structural mechanics degrees of freedom except segregated contact variables are solved in one segregated step.
In the COMSOL Multiphysics Reference Manual:
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Solution Operation Nodes and Solvers
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About the Stationary Solver