I was messing around trying to create a melon farm and when a melon stem grows a melon it changes the facing block state of the melon stem. The observer, for whatever reason, does not detect this. I further fooled around and found that when using a command block to forcefully replace a dispenser with a dispenser with the facing state at a different value, the observer can detect it. I thought that this may simply be that observers always detect blocks placed by command blocks, but repeatedly using /setblock or /fill to replace a block with itself did not trigger the observer, leading me to believe that this is just a problem with the melon stem.