are the test kit reagents ever wrong??

[topgunner19]

so tested some new tabs,, with Marquis and mandelin,, reagents, but not an instant black purple, but pretty fast say less than 10second reaction,, with a very subtle olive green tinge?? very little, just enough to see?? so figured they had MDXX and perhaps a bit of speed,,, these were tried later, and had no apparant affect, no buzz?? the kit works fine as proven in previous results and verified!!!! anybody have any clues to the mystery no show tabs?!?!