Pls help?: Mulberries are tiny!

Hello,
The weeping mulberry tree I planted 3 years ago, grows an annual profusion of berries, but they are so tiny, they are inedible. Even the birds can't be bothered, it seems; the tiny things fall to the ground. I remember the first year, they were small but at least pickable. Tree looks healthy otherwise. Picture attached.
I've researched this issue exhaustively, without success. You are my last hope. Please share any experience, or knowledge?
Thanks, in advance.
Lana

 

Is this tree grown from seed, or is it a cloned, named variety? Seed-grown trees can revert to wild forms with little resemblance to cultivated varieties.

 

Not from seed; I purchased it from a reputable nursery. First harvast was small but edible; then each year, berries produced were smaller and smaller.
Thank you so much for your reply!
Lana

 

How about your micro climate?
AFAIK, mulberries like really warm weather. Your winters are pretty cold and even summers might be cooler than mulberries like.
If you provide your mulberry with better winter protection and the best local micro climate you can offer, then maybe its fruits become larger?

 

I can't tell much from the single photo, but there appears to be a lot of crowded green shoot growth. Do you apply high-nitrogen fertilizer to the tree? Some plants produce lots of leaf growth and poor fruit production if supplied with excessive nitrogen.

 

I know some mulberries flourish in the local climate, but I may have a different variety, I suppose. I can try a burlap wrap over winter, but only until it gets too large.
Thank you so much for your input!
May have to be just an ornamental.

No fertilizer applied, but for a mulch of composted manure. The tree is quite young, only about 6 feet tall. I'm guessing that may be why there is a lot of new leaf growth.
Thanks for your continued effort to help.

No fertilizer applied, but for a mulch of composted manure. Additional photo attached. Thanks again!