Spool wire can wear out quickly for different reasons, particularly if the area being trimmer has a lot of debris (e.g. stones, large twigs, thick weeds. The string trimmer wire isn't strong enough to cope with the force of hitting the debris and consequently breaks. Care must be taken when maintaining edges of your lawn to ensure as much of any garden debris is removed from the path of the string trimmer.

If you have an automatic feeding string trimmer it is possible that the actuating lever, located underneath the spool (small triangular piece of plastic that acts as a ratchet to feed the spool line out) has broken one of the small plastic pins that affixes the spool to the housing. This would cause the line to feed out freely. If this happens the product will need to be taken into a Black & Decker service centre for repair.