Here are some ways that might help improve your canvas
1. Pull back the clear cover and re-smooth it back down again with your hand, this helps correct the mild creases.
2. Place the canvas under some heavy books - or a mattress as this can help smooth the creases out also.
3. If the adhesive has lifted from the canvas, just prick it with a pin and re smooth it down again using the cover. Do not scrape with a knife.
4. If the creased areas are still sticky, which they should be - the diamonds will still place well and the weight of the placed diamonds can help flatten the canvas out in the long run.
5. Never back roll the canvas - this will make it worse! If you are to roll it at all, roll with the picture facing outwards like the way it came.