Part of this is just the time of year. From Halloween through New Years, most raids, whether 10 or 25, easy going or hardcore, have more and more absences, RL pwns WoW.
That said, sometimes a preemptive "hey we know at least 4 people have company Christmas parties this weekend, no raid, see you on the 12th" on the guild forum or or GMOTD makes for a much less stressful non-raid than trying and stressing over people not being there/ trying to fill slots.
Second, the next patch with new content is close, you guys will have stuff to do. You can't fret too much over players lost, often when people don't fit well and stay in a guild it causes far more drama than if they had left at the point they decided it wasn't for them. Ever work hard to get a date with someone and then that hard won dates was just... okay? Not bad, but not great and certainly no sparks flew? Raiders who join you for a while and move on are going through the same thing, accept it and keep looking forward.