when you push the primer bulb it make positive pressure in the float bowl
and pushes the gas up the emulation tube.
One question when is it hard to start? when unit is cold, when it is hot ,
or any time you start it.
if you answer hard start when cold here is what is wrong

[1] - the gas you are using is old, gas over 30 days after you buy it at the
pump will start going stale the older the gas the harder it will become to
start, also the cooler it is the hard it will be to start as well. if you
use a fuel stabilizer to extend the shelve life of the gas it will elminiate
this and no fuel stabilizer wont make old gas good.

[2] - when priming (this is critical) you must push and hold the primer bulb
for 1 or 2 seconds, then release and wait 2 seconds before pushing again. if
you push the bulb fast you wont prime the unit and it will be hard to start
even it you push it 25 times.

let us know how you made out, P.S. to see if the primer is working when the
unit is running give it a push and the mower should run rough for a couple
seconds.