tightening a stretched canvas

First, has the room that the canvas has been stored in become more humid since you stretched the canvas? If so, the canvas may tighten up when the humidity drops. In this is not the case, unfortunately, without keys or some way to expand the stretchers, there is really no way to tighten up the canvas without doing some restretching. You may be able to get away with partial re-tensioning and not have to take the whole canvas off. You could try by beginning to pull tacks or staples in the center of each member and pulling the canvas tighter and then replace the tack. You can then do the same on each side of the 1st tack incrementally working towards the corners until the canvas is tight enough, hopefully you do not need to remove all tacks. I would not do this with an oil ground on linen but the canvas with acrylic ground is probably flexible enough for this procedure. Hopefully you left enough additional canvas along the tacking margins to facilitate this process. No matter what you do, including leaving the canvas in its current, less than drum-tight condition, do not use one of those proprietary canvas tightening sprays. We have seen examples where these have cause permanent planar deformations which cannot be easily remedied.