Sorry, it took awhile for you to get a response. It always helps to tell us your location. There are hardly any garden questions that can be discussed without regard to location/ climate.
A picture is worth a thousand words. Photos of the plant and the problem always help.
Bellflower is drought tolerant and likes well drained soil. So with all the rain (we having been having that where I am, too), it could well be staying too soggy. Plants can get root rot that way. Check the soil near your bellflowers a few inches down and see how wet it is. If it isn't draining well, is staying soggy, you need to improve drainage.
You can do that by mixing coarse sand, perlite, crushed granite etc, in to the soil around them or lots of compost and other organic matter. Or by replanting them higher (make like a little raised bed for them). Or by channeling rain water away from them. Etc.
Do they get enough sun? Any problems with too much moisture will be aggravated by lack of sun. They like morning sun and protection from hot afternoon sun, especially in hot climates. But they should be getting 5-6 hrs a day of direct sun.
I'm not sure what you meant by putting string around the bottom. If they are leaning or getting floppy you can stake them. If you want to spend money, you can buy grow throw plant supports, in various sizes and shapes:
Or you can just tie them to stakes/ sturdy sticks, etc. But be sure you tie them loosely, with something soft. Nylon knee-high stockings work great!! Anything that constricts them can cause damage, especially if they are blown by wind or something.
https://www.bhg.com/gardening/flowers/pe ... erennials/
I looked up your bellflower variety, which I wasn't familiar with. Very nice, bi-color flowers.
Welcome to the Forum! Let us know what you try and how it works...