No cherries on my Cherry tree

I bought a Porthos cherry bush several years ago and I'm still waiting for any cherries! It seems healthy enough and each year we get more blossom but this year there are maybe half a dozen tiny (size of a pea) cherries- the most we've ever had!
I understood it was self fertile and the only useful information I've managed to find is that it will have cherries "when established" - but no rough times are mentioned.
Does anyone have any experience of this bush - or any advice they could give to help me finally get a decent cherry harvest? I'd be very grateful if anyone can help.
Many thanks

 

The only reasons I can think of are frost at flowering time, cold, wet conditions putting off pollinating insects or lack of water making the small fruits drop.