That button is not in any enabled or disabled mode. Just press it if you suspect that DMA has reverted to PIO mode.

Now I'm just guessing.

Assume that the DVD you are using as input is a double layer disc. The first layer is pretty easy to read for the reader as you get normal reading speed, but the second layer is more difficult to read on just this DVD and as you posted that it happens at around 51%, when it then moves on to the second layer, that could very well be the case that it's difficult to read.