The only weaping I have encountered is with the Smooth-On 320 series resin. (and Smooth On owns up to this issue) It's a tan resin that is prone to seperation (in it's individual parts) The end result is you have to be obsessive about premixing the A+B components individually before mixing the resin, and then be aggressive with mixing the 2 components together. IF you do this it seems to work well, if you don't throw the piece out and start over. Ive actually sworn off the resin, which I love, because Ive lost to much to seperation.
There are other causes, but I thought I would share for anyone who uses the 320 series and who has run into this. The good news is, if you are consistent with the premixing, and mixing it is a very nice resin.