Why do you think the Night Watch went into decline? When Westeros was divided into multiple kingdoms it had far more strength then it appears to have during the Iron Throne period. Why by 300 AC is it spiraling into nothing? Where did all its usual recruits go?

I think there were a number of factors: 

1. The passage of time. After thousands of years with no White Walker attacks and increasingly spaced out wildling kings, the Night’s Watch seems less necessary. 

2. The establishment of the Iron Throne. Previous to the Targaryens, there were a lot more wars both between and within the Seven Kingdoms – which means you have a lot of prisoners after the wars who are rather inconvenient. Highborn might get ransomed, but it’s easier to ship the smallfolk up to the Wall and let someone else feed them. With the Iron Throne providing a continental state and increased peace and security, there’s much less prisoners to take because you have fewer wars and a judicial system to process people through. 

3. Interaction of inability to reproduce and capacity to produce.  Because the Night’s Watch have sworn oaths not to have children, you have to keep replacing members of the Night’s Watch or you start to decline. Moreover, since a third of the Night’s Watch is primarily engaged in feeding the other two thirds, when you start to lose men, you lose the manpower necessary to grow the food you need to keep your target number of men alive. 

4. Institutional weakness re recruitment and training. As I discuss in my exploration of Jon’s chapters in AGOT, the Night’s Watch doesn’t do a very good job recruiting people and training them. Yoren and his two fellow wandering crows simply don’t grab enough men to halt the natural decline in the Night’s Watch’s numbers, let alone reverse that trend. Moreover, there’s a huge bottleneck when it comes to training – Ser Alliser isn’t graduating enough people to make up for natural losses, and is not providing good quality training either.