There should be no problem with doing this.

With PSS, you need to ensure that the fundamental is consistent with the frequency you're sweeping. If it's a single tone, just put the same variable in the "beat frequency" field. If there are multiple tones, the easiest way is to go to the options form for PSS, and scroll down to the bottom where it says "additional options" and enter "autofund=yes" (this tells spectre to figure out the beat frequency from all the sources in the schematic).

For QPSS it always figures out the frequencies from the sources - so it ought to work.

You didn't say how it failed - so it's very hard to give a precise answer.

Regards,

Andrew.