I often kick myself for forgetting to slash the top too :D
If you slash it next time and it still doesn't rise as much as you expect, you might try leaving the dough to proof in a warmer place in your house (I put it where my hot water tank is). If that's not it then it's either a case of leaving it to proof longer (you can even put it in the fridge for a slower proof for a couple of days) or your yeast isn't good.
You would want cold butter and to rub it into the flour with your hands until it's like breadcrumbs before adding your liquids/yeast. I usually add about 15-20g (~ 1 tbsp ish) to a loaf with 500g flour. For the olive oil (1-2 Tbsp/15-30ml) I add it just before the water and reduce the amount of water by the same amount.
