Having trouble with higher pitches, or trouble hitting those lower pitches? Believe it or not it’s all about your vocal folds and good vibrations. In a nut shell, if you’re having difficulty singing high, then your vocal folds are not vibrating!
fast enough . . . . the need for speed. But If you’re having difficulty singing low, then your vocal folds are not vibrating slow enough . . . Go slow to get Low! 
There are different solutions for each of these speed issues but first and most important is to identify which issue you’re having in order to find the right solution. Simply if you’re having trouble singing high or having trouble singing low you’re really have trouble with how fast or
slow your vocal folds are vibrating. For example if you are singing the A above middle C your vocal folds need to be vibrating 440 times per second. If you go up another octave to the next A, your vocal folds need to be vibrating 880 times per second! How about that!