Need help with seismo chians/cancel and meterless fadc

So lately I’ve been getting really bored with akuma and I’m thinking about dropping him or making him my secondary character in ultra street fighter iv. I’ve always had an interest in viper but didn’t have the execution to play her when I first got the game. But now I think im ready to main her in USF4 I’ve been practicing with her for about a month and have learn her just about everything her tk feint , sj, normal canceling into u1, instant air bk,&FFF. But for some reason I cant pull off her meterless fadc & seismo cancel I need help with the inputs please help.

What inputs are you doing and what happens when you try to chain?

Normally, I do 623 for the first seismo and then 6239 for the next seismo. The timing is slower than you would think. I input the 6239 immediately after her fist hits the ground to create the seismo. If you sj forward, you did it too fast. If nothing happens, you did it too slow.

I do the same same thing when doing meterless FADC.

Super late reply but thanks for the help also how much do you think I should practice thd seismo chains for.