The waveform you see in GarageBand is the original representation of the recorded level. The higher the level, the "taller" the waveform. If you recorded a signal with a very low level, then the amplitude (the "tallness") is very little and it becomes difficult to edit.
Unfortunately there is no way to visually stretch the waveform vertically, for that you have to upgrade to Logic. There you have a feature called "Waveform Zoom" that lets you make the waveform appear "taller" by adjusting a the Waveform Zoom Slider.


To avoid that problem in GarageBand, make sure that the recording signal is hight enough, which is a good thing to do anyway for one important reason called Signal to Noise Ration "SNR"
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link