Homing grbl 0.9 problem

Hi, I am using Grbl 0.9 with Universal Gcode sender. All works well apart from $H the homing cycle. I have all the hard limit switches fitted and working. Have read and re read the Configuring Grbl 0.9 but when I press $H on then Machine control page all I get is the steppers vibrating like mad. I have $21 enabled. Any help please. Another problem, quite often I get the error "Error opening connection(java.lang Exception): No driver for port COM4. This port works fine normally and on a different Arduino sketch. I seem to suffer more problems with java than enough. (Hate It). Looking in device manager it says devise is working properly. The only way I get round this problem is to re boot the PC. Any help please.