If you do find that the scale ...

If you do find that the scale is calcium sulphate, you can either try to precipitate out calcium as CaCO3 in the pre-treatment as suggested by Anro, or alternatively precipitate out the sulphate using barium (e.g. barium carbonate) as BaSO4.  This is more expensive than adding lime to precipitate the calcium, but may be more suitable for your situation. This is also if the anti-scalents being offered still do not work.