Crystal oscilator not oscilating on a PIC
You might have too much capacitance on the oscillator pins this can be caused by:


Using the wrong capacitors for the selected oscillator/crystal.
Building your circuit on a breadboard which increases capacitance in the circuit.
Connecting a scope probe to the oscillator circuit.


When measuring the oscillator frequency it is best to connect the probe to the oscillator through a buffer. The PIC microprocessors often have a CLKOUT pin that serves this function, but it can only be used in some oscillator modes.