How do I replace the bottom plate of the power-mate?

check for a missing very small
"O" ring around the area where the canister fits to the stove
could be the seal where the canister goes in is damaged

Make sure vacuum is not plugged in to power outlet. Lay it upside down and remove 4 screws from bottom plate covering brush roll. Remove bottom plate and pull the brush roll out. Clean hair and debris from brush roll and make sure it spins freely. Remove end caps from brush roll if possible and clean debris out. Install new belt around motor shaft then insert brush roll into belt and pull on brush roll to give you enough leverage to stretch belt far enough to put. Brush roll back into position. Reinstall bottom plate and screws. Make sure brush roll will turn before turning vacuum back on. Belt will lose elasticity and should be replaced twice a year.

With the power head cover removed, you will see tabs on the inside of the chassis inline with the front bottom plate. Simply pinch the tabs with some needle nose pliers and push. I have included a link to the parts list as well as the front bottom plate. You will see by the picture what those tabs look like and how to remove the front bottom plate. 5411 HG Front Bottom Plate Windsor Sensor S12 XP12 SR12 86138280 allstar... http://sebo.us/pdfs/website_schematicsPDFs/AUTOMATIC%20X%20Schematics.pdf

plate has to be hitting brush try spinning brush by hand with vacum off and check for blockage

The top part of the nozzle pops off just put small screw driver on side and lift up under the top lid is 2 screws then pop off by putting screw driver on sides to lift the housing off base plate
here is roller
Sears Kenmore Brush Roller 8192670 8175102 And 742454
and roller

Sears Kenmore And Power Mate Canister Power Nozzle Belt Part 20 5285

If unit beater has stopped working it is either the motor, the power connection to beater or the belt that powers the roller. Which is it?

See if you can download the manual from the mfg or google for it.

Be sure to unplug your vacuum from the electrical outlet before proceeding!

-Turn your vacuum upside down so that the handle is resting on the floor and nozzle portion is resting on a work bench.
-Remove the 2 phillips screws from the metal bottom plate and set the bottom plate aside.
-Look at the right side of the brush roll. You will see a bent metal rod that attaches to the brush roll end cap. This is the clutch rod that allows your brush roll to spin or not depending on the position of the carpet/floor switch on the top of the nozzle. Pull this rod out of the brush roll clutch only. Leave the switch end of the rod where it is and just let the rod rest there.
-Pull the brush roll and old belt out and dispose of the old belt.
To install a new belt; put the new belt on the motor shaft first, next put the other end of the belt over the end of the brush roll and stretch belt and brush roll back in to place. Notice that on the brush roll end cap opposite of where the clutch rod goes, there is a metal piece with a slot cut into it. Be sure that this slot mates with the plastic stop in the nozzle so that the end cap cannot move.
Snap the clutch rod back in place and rotate the brush roll by hand to make sure the belt is not pinched.
Replace the bottom plate.

Push in on each side of the light cover on top and it will pop off. Then you can remove the (2) access screws. With the screws out the top cover will separate from the base.

I was able to get the whole thing reassembled.  If there is something specific that you are having trouble with let me know.  Everything snaps into its specific spot--it is just tricky to get it all lined up.