If the chlorine is too high, it will oxidize the sequestrant too quickly and will also oxidize ferrous iron to insoluble ferric. If the pH is higher, then the metal ions will precipitate or stain as metal oxides-hydroxides. Usually a higher TA level results in the pH rising over time, but by itself I don't see it interfering with the metal sequestrant (I presume you are trying to remove iron or copper and not calcium carbonate scale).
So the usual sequence is to first get the chlorine to near zero and the pH low and use a stain remover such as ascorbic acid for iron stains or citric acid for copper stains. Then one adds a metal sequestrant product and slowly raises the chlorine and pH to normal levels. One continues to add metal sequestrant product at a maintenance dose each week to replenish that lost by oxidation from chlorine. Metal sequestrants that use HEDP are more resistant to such chlorine oxidation (compared to, say, EDTA), but nevertheless they still get broken down.
The other approach to removing metal from the water is to do a water replacement after the stain is removed (i.e. after the ascorbic acid or citric acid treatment).
Yet another approach to removing metal from the water is to use CuLator, but if that is done and isn't working for any reason then the company needs to be contacted for special procedures. CuLator is not inexpensive, but if water replacement or ongoing metal sequestrant use is expensive or cumbersome then it's an option.
