I baked a rhubarb pie last night and it was a bit puzzling -- there was a slight 'crunchiness' of undissolved sugar in some areas. Not all the pie was like this. It's never happened before. It baked for the right amount of time, had a golden brown crust and the fruit was tender.
I've had this happen before. I've been told it has to do with the juiciness of the rhubarb and the amount of sugar. That is, the rhubarb juices absorbed all the sugar they could until they were saturated and the rest remained solid. I've cut down the sugar in my pie by about 1/3 cup, and it seems to have eliminated the problems.