I'm glad you asked, there must be a factor affecting your culture.
It is usually (there are many more complex factors at work but i won't list them here) a change in these factors temperature, nutrients, toxins and competition.
We can rule out temperature so that leaves us to nutrients and possibly toxins or some infections.
As nutrients the tibicos as any other microorganisms require a source of carbon, nitrogen and other elements depending on the species.
The sugar is the carbon source but it won't be able to live just on sugar, that is why people also add more complex foods for it like fruits and raisins which also have a source of nitrogen, different sugars and microelements.
My recommendation is try some molasses or unrefined sugars next time, just a spoon will do beside the normal quantity of sugar that you use and a few raisins, or dates. Some even use ginger, apples, oranges (without the peel).
That might make your kefirs spike, it surely did for me
As for toxic substances that affect the kefir, chlorine comes to mind.
I use regular tap water but i give it some time to eliminate the chlorine out (if your city uses chlorine for water purification).
Or just make your water reach a roiling boil and you are ok.
As for infection or competition with other foreign bacterias or yeasts i will leave this out sadly.
I have no idea how to eliminate the bad bacteria once it gets a hold on the culture.
Usually the kefir can naturally out compete any natural competition but if it gets weak due to bad nourishing it might collapse.
I heard some people will put them in plain water in the freezer and let them fight it out.
In theory this makes sense since if you give them nutrients the bad bacteria will always outcompete the good ones given the situation.
Good luck!