effect don't work

hello,

I am experiencing a weird issue, none of the effect but a hide and animate works.
has anynone experience the same thing, or has a clue where to start looking at?

I've been experiencing this on a div as follow

	$("#mydiv1").click(function () {
      $(this).animate({"left": "+=50px"}, "slow");
});

this works

$("#mydiv1").click(function () {
      $(this).effect("highlight", {}, 3000);
});

this won't. none of other effect as fade , shake, bounce

Thanks in advance for the help,

Will