People often find it easier to cry about smaller things than bigger things, because the emotions are easier to handle, so they can come out easily in tears. One of the famous essayists (I can't remember which, alas - I just remember reading a huge book of famous essays back when I was 19) wrote an essay about it - he told the story of some case where a king couldn't cry when his own son died, but he could cry at funerals of people he didn't know very well. The essayist was basically saying that it's harder to cry when something is so very painful and personal, because the emotions are too overwhelming to deal with easily. It makes sense, I think - it's very easy to cry in a 'tear jerker' movie, for instance, even though it's about fictional characters. But don't worry about the fact that you're not crying yet. You'll cry when you're ready - once you've processed the feelings.
I feel so sad for you when you told the dream where your grandfather was there and you looked around for your mum but you turned back he wasn't there. I can't remember how My grandfather looked like and I feel guilty about it :"""(