Hello, sorry for not replying sooner.
The related games algorithm is complicated, so it's hard to say why. Essentially it creates a graph of "signals" based on shared properties of different games (tags, buyers, raters, etc.). It then uses those connections to calculate a score, and the games with the highest scores become the related games. So there's no simple answer to your question about the tag overlap.
A possible explanation for not seeing many recommendations, the algorithm hasn't updated yet. It isn't calculated real time, but run every once and a while. So it may take some days before recommendations start appearing.
Hope that helps.