The Black & Decker weed eater uses a spring and a bump spool to automatically feed out more line when you tap the head onto the ground. Sometimes this head can get covered in sap or dirt, causing the spool to stick against the head. Overtime the spring can also get worn out or damaged from excessive tapping. Whenever the line stops feeding, you will need to clear the jam before using the trimmer again.
Most often the line will stop feeding out when it gets jammed up along the spool. The line needs to come out smoothly; and to do so, you need to wind the line in tight, even rows. The lines also can't intersect or overlap each other, which will cause the line to bunch up and jam. The replacement line for your Black & Decker comes in a specific diameter, so if you replaced it with line that's too thick, it will also cause the line to jam. Remove the head cover and all old line on the spool, replacing it with only the line recommended in your owner's manual.
Dirty or Sticky Spool
If changing and rewinding the line hasn't solved your problem, you'll need to check the spool to make sure it's not covered in sap, pitch, mud or dirt. Any obstruction will cause the line to stick against the plastic, so when you bump the head nothing will come out. You can wash the spool with warm water and a dish detergent.
If you notice cracks or holes in the spool when cleaning it, you've likely found your problem. The spool needs to spin properly in the head; and if the spool's cracked, bent or broken in any way, the line will not come out when tapped. Replacement spools can be purchased at any home and garden store that carries Black & Decker products because you will need to replace the spool with a Black & Decker spool.
Occasionally the spring underneath the spool will get bent or loose its springiness. When this happens, the spool will bottom out against the head and nothing will happen. Also the spring may not be connecting to the spool properly when you reassembled the head. Check the function of the spring by pushing the spool down against it; you should notice some resistance from the spring.