Did you try to add washed platelets?

I always use Aprotinin to keep the cells from degrading the fibrin

People will recommend aprotinin, but a cheaper alternative is aminocaproic acid (ACA). It inhibits plasminogen activators and decreases the action of fibrinolytic agents released by the cells in your hydrogels. Use at 1mg/mL media you're using to feed your cells; it doesn't entirely stop the gels from shrinking, but certainly slows them down.