Not an expert but...I would make sure you have fresh seeds and I would try stratifying some of them...some species need warm, cold, warm stratification so if they're fresh and haven't germinated, try leaving them as they are through the summer, fall, winter and see if they germinate next Spring. Another thing you might try...goes against conventional wisdom...collect them and sow them immediately w/no stratification. I found that stratification actually slows germination in spicebushes and magnolias! Redbuds could be similar! I'm sure you'll find the key if you experiment and don't give up too quickly!