The most important secret to making these crumb bars is to melt the butter and let it cool completely. If the butter is hot, it will cook the flour mixture and the crumbs won't form. I recommend melting the butter in a bowl in the microwave, cooling for 4 to 5 minutes on the counter and then pop it into the fridge for 3 to 4 minutes. It should not be solid but cool.
Make sure to drain the crushed pineapple from the juice. A little bit of it is okay but squeeze out as much as you can and reserve to drink later or add to smoothies.