I suspect this is because you have the VCD file driving digital nets. There is a long-standing limitation that VCD and vector files can only drive analog nets - and there's a CCR for this, 513270. You might expect that since VCD (Verilog Change Dump) is a digital format, it could drive digital nets, but the functionality actually comes from the analog simulator part (either spectre or Ultrasim). So it's a bit of an unexpected limitation, but that's how it is right now.
If this matches what is going on for you, the workaround is to connect resistors to each digital net and then drive those. Of course, that may impact your simulation performance if you don't have many (or any) electrical nets otherwise.
You may want to contact customer support and request this for yourself (please reference the CCR number) - that way a duplicate CCR can be filed to indicate your need.