It is hard to get a good bond when using epoxy and chopped cotton fiber in a clamped joint. The cotton filler tends not to flow when clamped. The clamping action squeezes out the epoxy leaving dry fiber behind. A weak joint results. The problem can be solved by using a filler that flows better such as Cab-O-Sil (my favorite) or glass micro balloons.

Also check the surface finish of the wood you are bonding. For example, if you used a power sander with fine or old paper, it is possible that the epoxy didn't have enough bite on the wood for a good bond.

Perhaps the wood you are using soaked up the epoxy leaving a dry bondline. You might try priming the wood with epoxy and letting it gell before the final glueing.

I agree with Mike, your scarf angle may be too steep.