I find that girth exercises make my dick kind of soft the following day; nothing to worry about, a day later it’s usually hard again, but I consider this a positive sign.
Think of it like this: When you clamp/jelq/stretch your penis in any way, you are stretching the blood vessels out. These vessels normally hold, say, 1 unit of blood (unscientific, I know, but just follow me.) When you stretch your penis, either horizontally, or vertically, you’re opening the blood vessels to hold 1.1, or 1.5, or whatever; problem with this is, your body isn’t used to sending that much blood to your dick. Give your body time to heal, and pump more blood into your dick. Jelqing, I believe, will help speed the process.
If you need an immediate fix, ie. you have some pretty-young-thing waiting to be plowed, try some amazing isometrics: take the head of your dick when you are as hard as you can get, push down towards the floor, and do a very hard kegel for about 20 seconds; this should give you a rock hard, slightly longer, slightly skinnier, functional hard-on. Hope this helps.