Why do my friend and I sometimes click, but other times it's awkward?

Why do my friend and I sometimes click, but other times we don't? For instance, we met up yesterday at the pub and had the best time together and clicked really well. We got along like besties. She then invited me over today and when I went round it was a bit awkward and the conversation was forced and we weren't clicking as well. Why is this? Is this normal between friends?

Why do my friend and I sometimes click, but other times it's awkward?
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