Several possibilities:- You are using a poor quality grout, the gaps are too wide for the type of grout (check the instructions - it should say), or you are mixing it too thin so when the water evaporates, it shrinks and the gaps appear.
Very probably. Personally I would never use a ready mixed grout or a DIY product. My favourite is Bal which comes as a powder you mix with water. They do a whole range of products suitable for all types of tiles and widths of joint. Stick to a trade professional product.
i'd put money on it that the only thing that you've done wrong is to buy an inferior product.
absolutely agree with plumbob - get yourself a decent quality grout in powder form that you mix up yourself. never never never buy ready mixed grout.
never buy this kind of building material from DIY stores - the pros don't shop there for this kind of stuff. go to the places where they go, ie trade tile shops. they will give you the right gear. and BAL is generally the brand of choice, and for good reason, its good stuff.
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