You might have an invisible parts problem in the assembly files. This problem occurs for two reasons.
Solution 1: The parts in the assembly files may have been deselected from the Assembly Navigator. If the parts in the assembly navigator list are blurred and the sign near the part name is grey, click the box near the part name. The sign color will be changed to red and the part will appear on the graphics window.
Solution 2: The reference set might be wrong. Change the reference set of the part as the “Model” or “Entire Part”. Right-click on the part then go “Replace Reference Set” from the opened list. The reference sets list will be opened as Model, Entire Part, Empty. Change the reference set as Model or Entire Part.