Dude i'll give you the best answer to your problem. While i agree in part to what everyone is saying here. I'll say its more psychological than anything!! The same exact thing happened to me,( although not with Deca , but with Test and tren,mast,prop triple stack). My dick was a fucking wood when i was making up with this nee hot ass fuck chick i had just met at college and when i whipped my dick out, BOOM! It fucking got soft! Why? Because of all the mental pressure that i had put in myself that i needed to impress this chick No matter what because she was so fucking hot! Big mistake!! Second time i went out with her i took things easy , we went for dinner then for a walk , took it back to her apartment, had a glass of wine,took a 10mg cialis as a back up, no pressure and i banged the shit out of her!
After that all the other times, no cialis no anything, my dick was a fucking wood bro.
So in summary, while i agree with most of the other responses here, my personal experience says its also GREATLY in part to psychological pressure.
So, just take half a cialis the first time so you feel more
Confident and you should be all good man.
Good luck !!