The more sun they get the better. Check the amount of sun light they get (at least 6 hrs.) each day.

Burning could be your problem. Raspberries and blackberries set fruit on year old wood. A green stem grows the first year, and blossoms form on it the following year. After fruiting, the stem may die.

Wild berries are left to their own devices and are never pruned. Examine a wild backberry clump during its flowering season. Tou'll find the viable stem with flowers, and the beginnings of new stems low to the ground plus dead stems. There won't be as many dead stems as one might think since many of these have already broken down.

What I suggest you do if you want to tidy up. Remove the dead stems after fruiting and pile for burning. Let the rest alone. Don't remove apparently 'dead' stems in the winter; Its hard to tell dead wood from dormant wood. The dormant wood will green up in spring and become the fruit bearing stems.

Both blackberries and raspberries require full sun most of the day. Hike a pasture infested with wild blackberries and note where they do do best. Duplicate these conditions in your cultivated garden. You'll find berries growing along small creeks and drainways, and some distance away from the margins of forested plots. Imagine the shadow line of the trees and take note how deeply inside this shaded perimenter the wild stems grow. This tells you the sun requirements. I have seen blackberries growing with small shrubs and scrub brush, but never within the full shade of a forest.

Blackberries will tend toward low ground where there is more moisture during the heat of summer.

A wild raspberry clump spreads by forming new plantlets. After the berries have matured, a number of green stems rapidly increase in length and bends to touch their tips to the ground. Left undisturbed, these stems will form new roots at these points of contact and a new cloned plant will start. After a few season of this, the clump will have increased in diamenter into an impenetratable mass filled with old stem loops fastened at both ends to the ground - a home a rabbit could love.