Hi Mb,
If you pour a small amount of fuel straight down the throat will it initially fire up easily ? If it does then the most likely culprit will be that diaphragm is made of cheap neoprene and is not working as it's meant to.
Many of the mower shops are starting to cheat by buying really cheap eBay parts just like back yard operators are doing now and still charging like they're using "GA spares" componentry. It's a price war between the internet and normal suppliers.
Cheers,
BB.