A few weeks ago I was mowing my lawn and my lawn mower seized up. This was my fault as it was low on oil. I changed the oil, sprayed some PB Blaster in the compression chamber and turned it over by hand. After that it ran great for the next few mowings. It's a Tecumseh motor.

Today I went to turn it over and it turned over partially and then got "stuck". I could turn it backwards by hand but not forwards past the spot where it was stuck. Even after removing the spark plug it still was stuck at the same place. Oil level was full. Not sure where in the compression cycle it was getting stuck. I tried the PB Blaster again (but didn't change the oil) and still couldn't get it turned over. Then I tried putting the mower on its side (like I do when changing the oil) and after a few tries rotating it back and forth I was able to get it unstuck. After that I was able to get it started and mow my lawn, when it first started it sounded like it might have some piston slap but after it warmed up it seemed to run ok.

Any ideas what would cause this?