OK, not a common problem for a threadline. There could be a couple of causes...
The first thing that comes to mind is line tension. If you are lucky you might find the recommended line tension for your reel online or in the service manual. But most manufactures don't state it.
Basically, the line needs to be put on the reel under tension. It is not a lot and for the most part just reeling in your line is enough. Give the line a squeeze on the spool. Thumb and finger on each side of the spool and squeeze. If you feel any movement then the tension is not tight enough.
You can purchase special line tensioners. But you can make one real easy. A bolt with wing nut (or normal nut) couple of washers and a spring.
Or you can do it manually. Put the spool of line in a bucket of water. Thread the line through the stripper guide only. The stripper guide is the one closest to the reel. Then wind the line onto the reel using your thumb and finger to apply tension on the line. The water will keep it cool and stop it burning your fingers. The water also lubricates the line as it is going onto the reel. Using only the stripper guide ensures there is enough water on the line as using more guides will knock the water off.
Did you stretch the line? Some line comes pre-stretched but it is a good idea to stretch it any way. You will need a park or other large area. Tie the line to a tree or post or fence etc, then walk out the line. Once all the line is out gently pull on it to stretch it. Be gentle, you don't want to snap it. You should be able to stretch it several feet. Then maintaining tension wind the line back onto the reel.
Over filling the reel can also cause your problem. Most reels will have capacity stated on the spool. If you are not sure, usually 2 to 3mm below the rim of the spool is good.
Some of the cheaper brand lines are just that...cheap. You get what you pay for! Some of the cheaper brands are just to hard and will not sit on the spool correctly. If you are not sure what to get, go to the experts. Ask your local tackle shop or BCF. You want the best and softest line for whatever your budget is.
Another thing I can think of is maybe your terminal end is to light. That is the bait, sinker etc is to light and not able to apply enough tension. After a few casts the line would become loose on the reel. If that is the case just use your thumb and finger to apply some pressure as you wind back in....you wont need to do that if you have a fish on
itsaboat mate...Life is just a boat and then ya marry one !