The Sodium bromide has created a bromine bank in your water. Every time you shock with chlorine the chorine is 'converted' into bromine until the bank is exhausted. Just keep shocking. This is why your chlorine is not holding. It is reactivating the bromine. I would be careful about overdosing on the sodium bromide because it can create a VERY LARGE chlorine demand. Just give it time to work and keep shocking unitl the algae is gone and your chlorine is holding. (and don't forget to BRUSH, BRUSH, BRUSH!)

BTW, this is not very different then running a spa on sodium bromide...sodium bromide is added to the water and then chlorine (usually bleach, cal hypo, or dichlor) is added to activate it. When the bromine level drops you add more CHLORINE to reactive the bromine!