There is no way to troubleshoot the problem without the files. I've imported many solidworks assy's in the past. They typically work.
If your files are proprietary, you can open a support ticket with PTC to have them troubleshoot the problem.
If your files are not proprietary, you can upload them to the community and maybe someone can attempt to conversion.
You could also ask the provider of the solidworks files to send "simplified" files to see if you can import those or tell them the import doesn't work and ask for them to export as creo files.
