Jan, the lovely lady who gave me the grains had also experienced similar problems. After a bit of net surfing I came to the conclusion that the little blighters had a mineral deficiency. Previously I used organic raw sugar, 1/4 of a organic lemon and a tablespoon of organic sultanas with the grains and water for the ferment. It was time to go back to basics, no more lemon, reintroduce half rapadura sugar (it's full of minerals) as well as adding 1/8 of a teaspoon of bi-carb to the water (more minerals if you are not using spring water). Thankfully the grains sprang back to life and began bubbling again.
Now if I want lemon flavour in the kefir I simply add the juice of half a lemon after I bottle the fermented liquid. Another method that I have been trying is a secondary ferment with crushed fruit or fruit juice. I make the kefir in the usual way, strain off the grains then add approximately 3/4 cup of pureed sieved fruit to 1L of kefir. Seal the bottle airtight and allow to ferment for another 12 - 24 hours on the kitchen bench. The longer you leave it the more effervescent it becomes and slightly more alcoholic too......hiccup! I've not let it ferment any longer than 20 hours as I'm a little worried that the bottle might blow as it gets very fizzy.
Mango flavoured water kefir - thank goodness for frozen mango from last season
After about 20 hours fermentation
If you try this don't be expecting it to taste like soft drink, it has a mild slightly fermented fruity flavour, I find it very refreshing. I'm eager to try some grape juice but I'll have to wait till they come in season, any fruit with a high sugar content would be suitable. I've also seen kefir made with freshly crushed apple juice instead of water. From my understanding you only consume small amounts of the apple kefir and you need to halt the fermentation before it tastes like apple cider vinegar.
Berry flavoured water kefir
After 20 hours fermentation
A word of caution, if you intend fermenting grains in juice or other liquid always use excess grains in case the fermentation kills off you kefir.
A few weeks ago I did a swap with the lovely Narelle at Just Like My Nan Made. Narelle was eager to obtain some kefir grains and as I always have excess I was more than happy to help out. In return Narelle sent me two chokos...yay and a bar of her handmade soap. Thanks again Narelle, I hope those babies are working for you.
Little chokos and handmade soap