It is very important to comprehend that you are not striking in the conventional way.
Tai chi is not like boxing.
You must not 'cock' the shoulder.
The power must come up and out - directed by the hips - inward to your opponents centre.
If groundpath is present, you should be capable of placing your hand on your opponent and delivering though without retracting the hand or cocking the shoulder.
'Moving qigong' will develop this ability.