It can be because:
there is not enough energy loading, and consequently not enough energy released by your brace.
if your brace has a full length footplate, and you have strong quadriceps muscles, you will not be able to bend your metatarsals enough. In addition, a full length footplate with a person who has good quadriceps, causes a tremendous amount of floor reaction. Floor reaction used in bracing systems is generally done for the purpose of creating an extension force at the knee, and preventing the knee from buckling. This is not a desirable force system to use with a person who has good quadriceps.