I have a Sears mower model 917.377795. While I was mowing the lawn today, I noticed that the height adjustment wasn't locking.
Here's a link to the parts diagram:
http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Walk-behind-lawn-mower-Parts/Model-917377795/0247/1500600/P0408071/00002?blt=06&prst=0&shdMod=917377795
And a link to the product page on Sears Parts direct:
http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Walk-behind-lawn-mower-Parts/Model-917377795/0247/1500600?pathTaken=&prst=0&shdMod=917377795

It's not the handle (part 18) that controls the adjustment next to the right rear wheel; that is locked in place. But the other wheels won't stay at height. It looks like the spring (part 17) is holding them at the correct height but the mower will "dive" at the slightest pressure or bump, then the spring will bounce it back up. The back left wheel (opposite the height adjustment) looks very wobbly. I've noticed that when I push down on the front wheels the arm that passes the height adjustment from the right to left side (part 35 on the diagram) seems to twist - the right rear wheel is stable, but the left rear wheel moves and that allows the front wheels to move as well. I'm guessing that a locking pin of some kind broke or something is loose inside the left rear wheel assembly.
By the way, it is not the cotter pin at the front wheel (part 39). I have had that break before and the front wheels lost their adjustment completely. This problem is affecting all but the right rear wheel. Any help is greatly appreciated!

I figured it out. The shaft across the back of the mower is supposed to make sure the left rear wheel stays at the same height as the right rear. I pulled off the left rear to see why it was wobbling and found that the weld connecting the shaft to the mounting plate for the wheel had broken. I have a new shaft/wheel plate assembly on order.