1994 Citroen Xantia electric sunroof

I only just got this car and the sun roof might have not been working correctly for a while - it lifts up and down, but won't open fully. The glass moves about an inch backwards and then stops, I can hear the motor laboring indicating that there is some resistance?
I would like to avoid a costly visit to the Citroën dealer.
Kind regards
George Tempest