It depends on the snake. One of ours will only eat if put the dead rat in his box and leave him in there with it for 45 min or so. Another one will only eat if we move it by the tail a little bit but with the rats face away from her so she thinks she is sneaking up on it and so on.
Each snake has its own likes and dislikes on how it wants to eat. There are times it will be enough to drive you crazy.
A few hints though
If you are using frozen make sure its warm enough or they won't be interested.
Feed at night in a darkened room
bang your head on the wall
put the dead rat in the box with it over a heat pad.
go insane, wait 2 or 3 months and then try again.