The plastic filter housing warps so the primer leaks and does not prime the carb
It works by depressing the float and flooding the carb, not particularly good idea but it had to keep the EPA happy and they want you to buy an electric mower .
Some times a new gasket fixes it, some times 2 new gaskets fix it & some times only a new filter housing will fix it.
When you take it off you will see the path for the air to travel along then compress the air above the float to push it down to raise the fuel level in the bowl so less effort is required for the fuel to be atomised & sucked into the engine to start it .
Less common is the retaining bolts coming loose but that will generally cause the engine to surge when it runs
