Curried Lentils With Sweet Potatoes and Swiss Chard

I found this recipe on Smitten Kitchen. I loved everything about it: lentils, curry, sweet potatoes (love that savory/sweet mix), and Swiss chard. Unfortunately it did not work out. The lentils, despite 45 minutes of simmering, never got completely soft. They were edible (and actually, everyone ate them) but definitely more al dente than one expects from lentils. I will probably try this recipe again sometime (the taste was wonderful) but I have a feeling that tonight's leftovers are going to end up as trash unless the lentils soften in the fridge overnight and I am not holding my breath.Anyone know how I can avoid this problem in the future? I buy my lentils bulk and my theory is that they've been in my pantry too long (maybe 2-3 months). How soon do I have to use them before they refuse to absorb water?

Here is a photo of everything except the Swiss chard and the toppings, simmering away.

Here is the red Swiss chard, with ribs removed but before being cut for the pot. Isn't it beautiful?