i have strong wrist and arm but the shuttlecock doesnt deliever with maximum speed everytime i smash im using vt80 and bg66u with 23lbs tension
I agree with Jacob mm. If you search some of my post, am currently using 22lbs on VTZF and 21lbs on ASZ, no issue. Both Bg66. I've also play with 30lbs tension previously. Even at 21/22lbs tension, overtime that tension would decrease and you need to restring eventually. Perhaps your tension have reach that point for restringing.
My understanding is that a high tension stringbed will require more energy to deflect it from equilibrium. So when it rebounds, it will release that energy into the shuttle. If the tension is too high however, then one cannot deflect the stringbed from equilibrium and the shuttle doesn't go anywhere. So what about lacking power on low tensions? I think it's the same idea as taut strings are more repulsive than slacking strings, though one could argue that 18-22 lbs. is already taut, but maybe it's considered slacking tensions on a microscale. Once I realized these factors were involved I tried to find a balance that worked for me, especially since high tensions just requires so much more effort to play well at.
None of what I've posted hasn't been scientifically proven and it's just based on my anecdotal experiences. I found that based on it I found my ideal tension in which I can clear end to end and corner to corner without too much effort.
For the OP, I think you should try a higher tension, try going 2 lbs. higher at a time until you're satisfied.