Plumber fitted Magna clean but plate heat exchange keeps blocking up?

Along time ago now the plumber fitted a new heat exchange for the hot water this resolved the problem for over a year, but eventually it clogged again so the plumber fitted another plate heat exchange which clogged after about three months so supposedly fitted a new heat exchange and a Magna clean but this cloged again within about three or so months. our plumber flushed the heat exchange outside and the heat exchange started to work again for a few months then cloged the plumber then came again flushing the heat exchange and showed me how to do it I then did it the next time, it clogs after about three months and cloged again I've just flushed it again, this always rectifies the problem but how come I have to flush it after a Magna clean has been fitted and how come it happened so many times after the Magna clean? It's a Magna clean professional and a vallant VCW GB 242E. 80 65 46 GB. 03