Robert -
Half the time? That's a lot. When it happens, it usually means that the link has been copied-and-pasted. That won't work. Why? The long answer is that sometimes a link is quite long. To improve the appearance, the software removes a bunch of characters from the middle of the link, and substitutes a short series of dots. The software stashes the characters that were removed, and when you click the link, it puts them back and the link works. But when the link is copied, it is the dots themselves that are copied, not the text behind them - and the link with the dots isn't correct, and doesn't work.
Aren't you glad you asked? It is actually a little more complicated than that, but the basic idea is correct. Let me try an experiment (below) to illustrate.
Bill
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