I work in a mechanical engineering field that uses orings extensively. Orings are used to seal off areas from oil/water, or to create a vaccuum. If there are no cracks in the canister or top, as stated by Kristen, turn then to the oring in the top.
For an oring to work properly there are alot of things to consider. Check the oring for cracks or tears missing from it.
Also take into consideration it's hardness. It should be somewhat pliable. If it is hard or brittle it will not perform because it wont squish.
It needs to be able to be squished in the groove.
Check to make sure it is round. If there is an egg shaped spot that was squished to much and took a set it wornt seal.
By "took a set" I mean it wont come back to it's original round state after the squishing pressure is removed.
If none of this is the case, then check the groove the oring goes in for any deformation. If the walls on either side of the groove has been bent in an area, and has a slightly bigger radius as a result, the oring will no longer seal.
It should be slightly loose in the groove with the top off, and then squish out and seal as you screw on the top.