Certified GKH Gardening Expert
It seems that you are just outside of the growing zone for this European plum. It is just a little too warm for them. This means that they will not get the chill hour requirements necessary to flower and fruit properly. This isn't to say that it will never flower or fruit, but it will be very rare if ever.
You will need to find a plum cultivar that is hardy to up to zone 8 or higher.
Here is an article that will help you with plum care, once you find a cultivar for your area: