I just purchased the 15'' rapid reload string trimmer. When the string gets short I'm tapping the trimmer on the ground but the string just stays the same length. The only way I can make it longer is by restringing it.
If you use a smaller trimmer line, it does not always release normally when you tap on the head. Release the trigger and flip it over to access the string spool. Note the makings for which way to load the string. Hold the base of th spool and push down on the string head and twist it in the opposite way of loading the spool . Then pull out some string from both sides. Repeat if necessary for extra length. This is a minor inconvenience and you may only have to do it two or three times for a 1/2 hour project.
I have the original 15" split shaft model and also a new PowerLoad model and both require a pretty firm tap to get the line to feed. As well, don't let the line get too short before attempting to advance it and be sure the head is spinning at a pretty good rate when advancing the line.