Take up spring? The one for the presser bar or the one on the tensioner? They are both a must to get the machine to behave. The fine coil spring on the tensioner is a must, even if it's bend or just a bit out of place it will mess up tension. All parts of the tensioner needs to be there and in reasonably clean condition to work.
If the machine gets stuck it is mostly likely still oiling and cleaning. Especially if it's not quite run in yet after long term storage. Detect all oil points, investigate thorughly, make sure oil seep in to the inners of all moving parts where metal touches against metal. I'm sure you are aware of them; but just in case; there are oil points on top of the machine, holes in the black cast iron parts, there are a few joints behind the round plate in the back. There's several under the base and the actual points are at the end of rods, and tiny holes to reach inners of joints and hinges, a cam like part the feed dogs roll on. Behind the face plate all joints need oil, and make sure the rods for presser bar and needle bar are oiled where they enter the cast irion part and comes out in the needle and presser foot end. The hand crank have oil points too, but I have hardly used a hand crank and perhaps other can advice on it.
To get it right with these old machine you often have to take all plates and covers off, clean and oil tenatively a few times. Lint, grime and old oil gradually loosens and dissolve. Scrape grime and dust out of feed dogs, every corner and groove should be clean, and the same for the bobbin case and hook parts. Even when you think you have done it more than thoroughly, it's easy to have missed a spot. It takes time to flush all grime and stickly oil out of joints and hinges in a model 15. It's usually all about tentative oiling and cleaning, daily test sewing and oiling for the first week or so.
Other things to check; needle in the correct way. Does your model have a stitch length lever with revers stich; or more of a screw with nut at the end and no reverse?