It was only about 7 years ago that my husband and I had a great group of friends. This was pre-kids and we spent most Saturday nights together at restaurants or clubs. It was a great time in our life, but then it all ended when a couple of friends suddenly split up, one couple moved and another couple married.
Of course people move and we are still very good friends with that couple and holiday together regularly. However we now don't see the other friends, although still friendly with most of them.
One couple split due to infidelity. It was a very stressful time for us as this was my husbands best friend and he regularly used my husband as his alibi, unbeknown to us many times. At times it was known to my husband though. It made him incredibly uncomfortable and as we were all very fond of his friends wife and although he was very angry with his friend for cheating he also did not want to be the one to "break up the marriage" if he said something. I was in the same position. Finally the truth came out for the most part - certain truths were never told. The couple split however no one remained friends.
Now we have a whole new group of amazing friends, but there are certain couples like ourselves that make a big effort and are always initiating catch-ups, etc and certain couple that attend these catch-ups but don't initiate them. The sad truth is that no matter if you move or have problems in your life to maintain a close friendship you have to make an effort. Of course there are the friends that you can go six months without talking to and pick up the phone and feel like it was just yesterday, but those are few and far between.