Stuck Starter Rope
A pretty common cause for this particular issue the engine flywheel brake is engaged. This is the bar you hold on the handle that stops the engine when released so just make sure you are pulling it down the whole way before attempting to start the mower. Another probable cause is the blade is clogged or is dragging in the grass. To sort this out disengage the spark plug and move the mower to a hard surface so you can clean the blades to try again.
Mower Will Not Start
Lots of people experience this particular issue and the extremely common and most obvious reason is that you have no fuel. As well as making sure its all filled up also remember that old gas can also cause problems so if its been a while since using the mower it may be better to empty then refill the tank. Other possible causes are dirty, broke or disconnected spark plug, dirty fuel filter or fuel isnt reaching the engine.
Losing power while Mowing
There is a few reason this can happen and they are normally pretty easy to fix. Try cleaning or replacing the air filter or raising the cutting height on your lawnmower. You could also try cleaning or replacing the spark plug or clean the underside of the mower.
The Mower is Cutting Unevenly
If you are seeing this happen there is a chance you also feel some motor vibration. This can be caused by build up of debris or clippings and just needs to be cleaned. Otherwise you should try sharpening or replacing the blade.