When Paul and I use flags, it is usually to prevent pigeons from landing in areas that are too sensitive to shoot such as near public footpaths or dwellings.
To be effective, the flags need to be large enough to really flap in the wind.
However, they do not necessarily push pigeons to where it is convenient for you to shoot them.
If they are hell bent on feeding in a particular part of a field, flags will not stop them for long.
The solution will always be to set up where your own eyes tell you the birds want to be.