What Viper you have? There is bunch of 'em. Not from the same company (Elbox, DCE, Apollo, others).
Some (2) have provision to solder a second SIMM slot. If your board is the DCE Viper mkII, check just for cold joints in your new SIMM slot.
If the M-Tec E-Matrix (also rebadged DCE Viper MkV), look if you install the correct SIMM holder.
