Loose jack
 

So when I plug the cable into my Warwick Thumb, the jack doesn't hold it in place. There's almost no resistance. What do I do? What's wrong with it?

Sounds like the jack on the inside is bent or broken. If you have no soldering skills take it to a shop for them to replace the jack. If you know your ABC's about soldering then get a new jack and put it in.

Open it up and take a look. If the hot clip is bent, just bend it back. It's very easy to see where it engages the notch on the plug

Yup, I'd try this first.

All good advice, unless
 

your bass has a barrel jack, in which case you can't do anything but replace it. This would be a good time to think about modifying the instrument, so that you can use a decent 3 buck Switchcraft jack; when I replaced the jack in my Cirrus, I went with an electrosocket.

Barrel jacks stink. Mod it out to a Switchcraft old school jack like JLS suggests. Or put a new barrel jack in and replace it again within 5 years, maybe less. They are kind of like a timing belt on the car; change it on a schedule or it will fail when you least can afford it to.

Not as easy as saying it. :eyebrow: Take a look at the Thumb and consider what would be involved in your suggestion.

OP: buy another barrel jack.

I was supporting JLS's suggestion :eyebrow: he suggested an Electrosocket too. So yes you'd have to have the hole enlarged: any good luthier or woodworker can do it easily. A worthwhile mod for a nearly bulletproof result. It was only an alternative and a valid one at that. JLS is not a neophyte in the bass repair world or TB; his advice is never random.