I feel your pain, there is a new trend in so called 'white thread' that is a polyester, and the white that you initially see is the wax they applied to the thread, but, when you start sewing, the wax comes off and leaves that bland looking polyester see through thread that does not give us that pop that the old white thread would do !!!
I am currently in the hunt for some thread, my old thread came from Weaver Leather but when I recently bought some from them, it was NOT the same and I returned it, Tandy was the same and I tried a different place, same thing. Today I got a roll of thread that kept it's white look but is pretty thin so not great but maybe passable until I locate a good source.
Here's a shot of a sheath I did a few years ago, the white really pops and makes great contrast
sheath_cityknife by GaryWGraley, on Flickr
here is one with some thread that the 'white' rubbed off during sewing, you can see that the polyester is what is now showing, not near the pop, holds secure but doesn't have that nice contrast I need.
Tribal Lock by GaryWGraley, on Flickr
IF I can locate a good source, I'll leave info here if that is allowed.
G2