It may just be the spring weather, small runoff, and a few cold days over the weekend. I also had a slow weekend.
Boulder creek is usually dead easy ( I learned there). I can usually count on at least catching a few small browns.
But, I did get the skunk the other day on BC right after the weather cleared.
With faster water, you might have the right flies but not the right depth, Fish could be holding tight to the bottom and right next to rocks in eddies. Split shot can help, just make sure your flies are tagging bottom occasionally.
Or, use a longer line between the dry and dropper.
Sometimes if you start walking the river you will find a pod of fish all holding in the same spot. If you saw no fish, good chance they are all hanging together in a deep slow spot. I have found pods of ~100 fish on BC when the flow conditions start to change and they have not all spread out yet. A few weeks ago I found a pod and landed ~12 in about 2 hours. Went back to the same spot later that week twice and got nothing... just gotta walk and find them. Streamers are great for locating fish because even if you don't get the take you will see them chase and swipe at it. Find them, then nymph them. Change depth before you change your fly.
Fast flows, I start using all the flashy stuff that has been hiding in my box all winter. Also, even if you don't see any, a big #8 stonefly nymph can be a good attractor ahead of a smaller fly like your caddis pupes.
I fish almost daily for maybe a couple of hours, send me a PM if you are around Boulder or heading to the T, There are a couple of other sweet spots that are between. Also, the warm water bite has been great for perch, bass, walleye, gills, carp, etc. A lot of people don't even bother fishing runoff (although I think it is a fun challenge, and usually without crowds).
Back to my point: Spring is erratic, I've had stellar days and skunk days. It is always easier to catch fish when you are enjoying yourself though... it is as if the fish know when you are taking it too seriously!