I have the same problem with my wc vipers...often staying on the ground and not eating voluntarily...what i do with my mangrove viper is i put a dead mouse inside and cover the enclosure....the next morn the mouse would be gone...
however..this didn't work with my hagens and wagler...they strike but don't hold on when i tease them with a mouse...
try assist feeding them...a bit dangerous..but better than force feeding...