any specific reason that could cause this?
Not sure if theres any specific reason but from my own experiences I have also seen changes in volume and power of spurts , and can only guess at reasons for this..
where I live it gets really cold in Autumn/Winter and very warm in summer.
when it's hot weather I generally feel hornier and my ballsack is really loose.. my balls slap about which adds to the pleasure and the cumshots are usually bigger spurts and the spunk is of much thinner content..
When the weather is cold my cum seems to be much thicker and I rarely manage more than one spurt , with the rest of my load just trickling out..
Also , spending more time staying aroused before cumming makes a difference for me.
I find if Im edging for at least an hour I usually cum better.
Volume of cum could be affected by how long since your last cum , and how turned on you are at the time of spurting..
If it's sex with the same person that you always have sex with , or are just wanking to porn which you've already seen before then it would be less of a turn on , and affect how intense your orgasm will be and how powerfull the cumshot is..
Sometimes I just masturbate because I want to , and go through the motions which always ends in a poor cumshot , but lately I've been watching live amatuer webcams and have enjoyed better cumshots this way , but when it stops being exciting I will start watching something else to maintain a high level of arousal. Solo masturbation videos or group sex , or outdoor sex videos keep me horny..