Well, there's a trick to it. You have to let them know you are fixing things. It's even better if they know everything that's broken and can prioritize what they want fixed first.

I went through a lot of pain before I realized I needed to toot my own horn more. After that, suddenly they were very grateful that I was fixing bugs in their application. (To be fair, they were very grateful at first, too, when the bugs were really obvious and costing them money daily.)

There's no harm in letting them know exactly how much you've improved their system. They really will appreciate it.