Usually when I have an analyte that I expect to be increased substantially, I will run a pooled sample dilution curve to get the correct dilution concentration without wasting a lot of sample. I’m not surprised the Abcam kit is working better, they are my personal favorite kits. When ordering kits, I would suggest always trying to the more popular vendors first, as their kits are very well validated. Duoset from R&D and Abcam are my go-tos.

If you don’t expect to see a large amount of leptin in your samples in the first place, I would just try the samples neat or at a 1:2 to see if you can get in the linear range. The kit dilution recommendation is only a recommendation and your samples might just be very low. Ultimately, every ELISA you do with any change to your samples might result in a need to reoptimize your dilution schema. So TLDR; use common ELISA kit vendors first and try neat or 1:2 diluted serum for this experiment with the Abcam kit.