My MCD shades did something that sounds like what you describe. One windshield shade would move a half inch at a time but the other would not move at all.
The cause was a shorted motor in the shade that did not move. When it was disconnected the other shade worked fine. The MCD factory used a pretty small wire size and when the shade switch was pressed I think the voltage drop on the power lead to the shades was so much that it screwed up the other shade.
Clay (WA5NMR), Lee (Wife), Katie & Kelli (cats) Salli (dog).
Fixed domicile after 1 year of snowbirding and eleven years Full Timing in a 2004 Winnebago Sightseer 35N, Workhorse chassis, Honda Accord toad