Ask Mr. Smarty Plants

There are many reasons that your tree may not be fruiting as abundantly as it did in the past.  You did not mention whether or not there has been a lack of flowering as well. Generally, dogwoods are not considered to be alternate bearers like apple trees are.

If it is not flowering, it is likely a cultural issue; either not enough nutrient, water or sunlight.

If it is flowering, but not fruiting, the issue is somewhat more complex. Perhaps the flowering cycle was "off" due to an early spring and the necessary pollinators weren't around yet or perhaps the general population of pollinators is down.  If the fruit is setting but not maturing, that is yet another issue.

Although I cannot really answer your question, I hope I have given you enough ideas that you may be able to figure out if you have a problem that can be remediated.  I highly recommend you contact your local agricultural extension service.  They will be able to tell you if there is a regional issue and will be able to diagnose a disease or cultural problem if you can take them a sample or a more complete description.

Agricultural extension is organized somewhat differently in Canada than it is here in the US and is provided by the agricultural colleges.  The easiest way to locate the service is by determining if there is a Master Gardener's association in your area.  They can point you to the help you need or recommend a reputable nurseryman in your area.

In the meantime, I hope your flickers find an alternate fruit source ... I have a Pagoda dogwood that was stripped bare by the birds about a month ago!