Grainy like you can feel the powdered sugar when you taste/touch it? If so, that has happened to me if I've used a last resort el cheapo PS because I've run out of the quality stuff.
If that's not the grainy you mean, maybe you can post your recipe to see if anyone can help.
Did you sift your Powder sugar? I always sift, I buy mine at Sam's or Publix brand. And another tip, I have notice that when I make a double bacht it comes softer, someone here mention that the buttercream will be better if it covers the flat beater when you are making it. HTH
CIAO
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