Dirt Devil 12 amp featherlite vacuum doesnt work like it used to work like. I dunt know how to fix the problem?

Those have 3 filters and all three need to be cleaned. Look it up on the manufactures website or in your book. Also check the suction hose coming from the agitator to tank for blockage. It has to be disassembled to unblock or use a coat hanger. Unplug first and be safe.