Trouble planing long boards

I am trying to understand why I ran into a spot on a long board where my plane would not cut. I am making an 8′ long shaker peg rail using a 4x8x3/4 length of oak and need to smooth the show face. My bench is only 4.5 feet long, so I put the board against a stop to the left side of the bench and clamped the right side to the bench. To smooth the face I started at the stop end and started working backwards. This went well until I got about 2 feet back and the plane stopped cutting. So if I started at around 2 feet back it would not cut until I moved it forward where it would catch and cut again. I tried starting further back and it still didn’t cut. I reversed the plane and tried to plane away from the stop toward the problem spot and it still didn’t cut. After that I tried moving the board and starting the plane at various places, but still no go. Planing at a 45 degree angle across the face did cut some. A straight edge on the board didn’t really show much. I could see that at that point the board bowed up somewhat from the bench, but whether I pushed down to flatten it or used a gentle touch still did no good. As I moved the plane in away from my stop and toward the problem spot I could feel a change, as if the board got thicker, but the plane still didn’t cut. It was also odd that the spot that wouldn’t cut was longer than the plane. I tried extending the blade more and could get a cut, but then it would reach another spot and just dig in. Finally, I got out my scraper card, established new burrs on it, and smoothed the face that way. But I’m still puzzled over how to figure out what is going on when something like this happens and how to correct. I assume the board  is hollowed at that point, but even starting well away from the spot I couldn’t get the plane to cut.

Todd