I don't really see a difference either. But sometimes we can see what others can't.
Anyway, if she won't buy it, why not just buy it yourself? Suave sells one that is about $1.. It's awesome your mom is into gentler shampoos and what not, but moms are also stubborn and always "know what's best." even if they really don't. LoL!!
If nothing else works, just buy a harsh shampoo and scrub all that build up out. If that doesn't work, then it's probably not build up. But you have to try first. Start eliminating products that you think may be causing this build up if it does work.
You'll get it..