I've often noticed that planes flying on more or less the same track, headed in the same direction leaves different contrails. By different I mean how long they stay visible and the length of them. Why is this so? Is it weather related?
It is a mixture of factors. All else being the same:
If there is not enough (relative) humidity, the contrail will not form at all. See for example this image: Image from NASA
The contrails start forming on the east, near the clouds where you have a higher humidity, and do not form on the west, where no cloud is present and you have higher temperature/lower humidity.
What you observe might be airplanes flying at different altitudes and thus encountering different humidity/temperature combinations, giving birth to contrails with different histories.