The (main) reason toasters do not stay down as they should is because they all (by safety legislation) have a safety feature that makes the toaster pop-up if the toaster is short-cuircuting. By holding the lever down, you may be running the risk of electricuting yourself.
Often if this is the case, you may notice that only some of the strips within the toaster are lighting up or another indication, part of your bread is not 'toasted'. You can pull the toaster apart and and see if you can identify where it is shorting out (often a bug or something else lodged between the casing and the wires) and dislodge whatever is causing the cut-out in power. *make sure you unplug it from the power before doing this though*.