I'm having some problems when opening some workplans from PPMC on MS Project 2007, in this case I have a plan with 506 lines, when I open it on Project 2007 it opens without identation on the tasks and also after line 251 it appears blank tasks.
When the work plan import starts but isn't completely correctly imported, it means that something went wrong in the middle of the import process.
Please always provide the PPM version you are using ; Following steps apply only if you are using PPM 9.1+ or 8.02+ with the hotfix for MSP 2010 support.
You can enable the logs in the MSP Plugin (from the User Preferences menu), try to import the project, and then check the logs. The cause should appear in file msp.log. If you have any issue to analyze the logs, feel free to attach them to this thread.
Most of these issues are caused by invalid data (for MSP) coming from PPM, but the logs surely will tell us more about it.