Sounds like one of three things: Not enough hardener (Technical term-too low of a cure agent to polymer ratio), bad epoxy, and not throrough enough mixing. Most likely the third since it appears you got no curing at all. Mix the material really well. Time how long it takes. Go back and mix some the material some more for that amount of time. e.g. When you think it's mixed enogh' keep mixing. I use a metal spatula and spread the material out on glass; scrape it up and spread it out again. Keep going with the process until the mixture looks homogeneous. Some material appears to have "waves" in it if not blended throughly. Mix until these "waves" are gone. Then, mix some more. For small batches I use strong toothpicks on PostIt papers using the toothpick like a spatula. Use white paper if you're very fussy about initial results. If not coated, the 5 minute stuff will turn yellow after a while anyway. You won't notice it on a Copper John, but you will if you do large saltwater flies.

If you are using epoxy packaged in attached two syringe configurations with the plungers attached at the top; put a thumb on each plunger and pay attention to the amount of material coming out of each nozzle. Vary pressure to the individual plungers to keep the amounts coming out of both nozzles consistent. When in doubt, too much hardener is better than not enough, but don't try for excess hardener. Cure agent to polymer ratios also effect the cured materials hardness, you may wind up with a material that is too brittle. Not enough hardener can lead to soft material (in addition to the stuff not curing at all) with a tacky surface. If this happens and the material is soft and tacky, but structuraly ok, coat it with head cement. Most commercial mixes on the market let you get away with a lot of mistakes in measuring as long as you have thorough mixing.