What type of string are you using? Sisal will rot pretty fast. Plastic/poly string lasts a very long time outside.We have made square bales for 30+ years. Then we decided to buy a round baler. We liked the fewer moving parts. However, the strings on it fall off the ends of the bales. We leave the bales in the field longer than the squares because people told us it took longer to cure and we didn't want to burn the barn down. It was easy to relax vigilance on how soon we hauled them, but frankly didn't know how long we were supposed to leave them in the field. Now we are wondering if strings typically fall off a lot on round bales or if it is because we left them in the field so long. We had a bunch of unusual rain events, and ended up leaving the bales a long time. We are also wondering if the netting is easier to use than string for this reason. I had to run around on the 4-wheeler and rewrap the bales the other way, and that still didn't help much.
If the string is truly falling off at the end of the bales, change your tying pattern to move the last wrap back toward the center of the bale and make sure the string overlaps to ‘tie’ the twine.