When I was a beginner I thought the very strict instructor was
just being over cautious about cleaning the pallions of solder.
I cut some extra and stored them with an anti tarnish sheet in
a film container overnight. The next day I couldn't get them to
flow for the life of me. I quickly developed a firm respect
for Dr. McKays advise.
Hmmmmm, I learned to snip sheet solder, but have graduated to
wire. Anyway, I have found, that if I dip solder (either sheet
snips or wire) into the flux, I have no problem with flow. You
may want to think about storing your snipped pieces in flux -
that should prevent tarnish . . .
At any rate, it sounds like you may not have gotten your piece
HOT enough for the solder to flow properly. I haven't
experienced any problems with "tarnished" or "untarnished"