The wall looks like it is constructed of lightweight blocks which will struggle to hold a heavily loaded shelf with just two fixings. If you imagine the loading in your mind you will see that the downward forces will be forward of the fixings putting a downward and outward force on the fixings. The lightweight construction blocks are built for downwards load bearing and the softer fabric of the block probably cannot contain the forces exerted onto and outward from the face.

You may have to think a bout new brackets with more fixing points or a resin filler kit to beef things up. Some of the other pro's may suggest an alternative so hold tight.

DWD

I wouldn't try to re-use the existing holes. Whatever you fill them with it will never be as strong.

On Celcon blocks (lightweight blockwork) I find it best to use a long diameter screw (#10 or #12) and put long screws, 3in to 4in long directly into the block without pre-drilling or plugs (this does take a good driver). The more screws you can get in the better which means it that may be preferable to cut a couple of pattresses (planks), say 3 to 4in longer than your brackets and say 4 to 6in wide and fix each to the wall with 4 or 6 screws. The brackets are then fixed onto the pattresses so that the shelf rests on the top edge of the pattresses as well as the brackets. I've hung quite a few heavy steel electrical control boxes and steel sign boxes this way without any problems to date. The same approach works with heavier cement block, although for those it is necessary to drill 7mm holes carefully and use brown plugs (sometimes two or even 3 in a hole) to get a decent fix