No..its not the strainer or any other equipment..as I've always used same for the job.
Some kefir 'colonies' really do behave differently as much as they behave differently depending time of year (but thats to do with temperature).
These tiny Kefir grains that never seem to grow much bigger tend to result more fizzy type drink rather than the creamy, thick sort and there is big difference with flavour too. No matter how long I nurtured them, thinking I might 'turn' them to behave differently..they just wouldn't budge. It must be the way the bacteria and yeasts in the matrix have developed. The 'nice' colony that I had grew almost up to apple size before it naturally started started to fall apart.
I once even bought some from Kefir shop on net..and those were those were 'nice' neither.
There used to be world wide Kefir forum and it was from there where I got the 'lovely' ones. The last grains of those got spoiled with Viili culture which is notoriously 'infectious' and even if kept in same room for some mysterious way find its bacteria into kefir...resulting hybrid strength kefir that is much more gelly like (bit like viili) but in such of way that instead of spooning it like yogurt (like you eat viili) you ended up stretchy and ropey goo and you had to put your teeth into. Taste wise it was fine but it sort of stuck into throat when you tried to eat it
Once the kefir gets like that there is no way of reversing the colony..the viili is and stays with it..