The brain can only try to understand if the ear hears it in the first place. It will take her brain a good six weeks to six months to learn to hear as well as it can through new hearing aids. Some people need a moment to process what was just said, so slowing down your speech and speaking to them in a quiet room will help. Try to reduce extraneous noises such as the television, radio or other conversations. You may also try gaining her attention like saying her name or tapping her on the shoulder before you start speaking.
Some people can hear and understand in background noise better than others. There are speech in noise tests which can tell us who needs the most help in background noise. More general advice is to get hearing aids when your hearing loss is mild to moderate instead of waiting until the loss is severe as in general the earlier you obtain hearing aids the better.