Is it covered up and lying on bare earth ? If not, it should be to allow for worms etc to get at it. Mix it well, do not dump grass in large lumps on a compost heap. Mix thro some ordinary soil into it. Compost accelerators are useful, but not really necc. There are 2 processes going on in a compost heap, small animals (worms mostly) are munching thro it and there is bacterial action, both are necessary. Ordinary soil contains all the bacteria you need. Every time you top up the heap add a small amount of soil, or even better, take a small amount of well rotted compost from the bottom of the heap, and add this.
PS: Do not compost grass that has had weedkiller put on it. You can tho put this in brown bin where it will be mixed enough to negate the harm these do.