I never had such problem so I'm not very familiar with this topic, but the first thing that came to my mind was pixel rounding project property. Do you have it ON?
From the manual:
By default Construct 2 objects can be drawn at sub-pixel positions, e.g. (100.3, 200.8). If the browser is using linear filtering, this can make fine pixel art appear blurry. If Pixel rounding is set to On, Construct 2 objects round their position to a whole number before drawing, e.g. (100, 201). This prevents any blurring, and can also prevent "seams" appearing on grids of objects. Note this does not affect their actual X and Y co-ordinates, which can still be between pixels - it only affects where they are drawn on the screen.