question about my troy-built push mower

Hello all. I have a troy-built self propelled push mower. I bought it used because I needed a mower quick since my old one died. I have never had self propelled before. the mower runs great and mows good. my question is when I have the self propelled engaged it goes forward just fine but as soon as I let go of the lever the front wheels will lock up. I do not know if they are supposed to be free wheeling when not engaged or lock like that. when I pull the mower backwards I release the lever and push down on the handle to raise the front of the mower and pull it back then drop it back down and go forward again. I know this is probably a stupid question but I would like to know since I have never had self propelled before. the model # is: 12A-566N711 and the serial # is: 1D294K00212 thanks.