Welcome to FM.
All of the above plus, on some venues, by introducing fairly large quantities of groundbait, sometimes the fish will move up in the water to intercept the bait on its way down. This is particularly the case on deeper waters. If you are catching loads of fish on the bottom, and it suddenly goes a bit quiet, it is certainly worth trying to shallow up a bit. Just keep altering your depth until you make contact with the fish. If you don't make contact, go back to bottom fishing and try a different bait. Brandlings and Dendras work well for bream and will catch other species too.
Conversely, introduction of too much, and sometimes any, groundbait can kill a swim stone dead. I recall winning a Winter League match on the Basingstoke Canal once when the anglers on either side of me "balled it in" and I doubt if I used a quarter of a pint of casters all day