20 MHz crystals are usually fundamental. The change-over to overtone is nearer 30 MHz nowadays.Does it not start reliably, or is it drifting?
Does it oscillate at approximately 1/3 of 20Mhz?
20Mhz crystals are usually third-over tone type, meaning that the crystal has to be forced to oscillate on its overtone, while it prefers to oscillate on its fundamental freq, which would be about 6.6MHz for this crystal. Overtone oscillators must have an LC tuned circuit in them to force the crystal to oscillate on its overtone...
I think you will find that in your circuit, the crystal will be oscillating at its fundamental.