I can't recall much of the deleted post, but some of your slippage problem might be more about the coatings on the fishing line. UHMWPE fibers are very slippery in their raw form, and the mill that spun your fishing line may have treated it to some degree or another with a treatment that makes for good fishing line, ie to cast well, but maybe not the best for splicing.
Even if the line was treated or coated with a different coating more specific to splicing and knot holding, you would likely find that special knots or practices are also recommended to help overcome the inherent slipperiness of UHMWPE.
I would experiment more with your splices, maybe make them longer or maybe even try to sieze the splice with rosin or even try to wash the splice area with solvents to see if you can reduce the slippperiness of the fishline, if it was coated with something not to your liking.