Anytime you are dealing with a mixed mesh that contains beams you must define contact sets between the beam elements and the solid or shell elements. In your case you say solid plate but then you refer to a shell mesh. Do you want your plates treated as shells or solids? You can change that in the analysis by finding the member in question and then right clicking on it in the Tree and choose to treat as solid or shell depending on how it is being treated by default.
These types of problems can be very difficult to troubleshoot without seeing the model so if you want you can upload your model and that might help some.