I don’t know if that promise applies to Maaser K’safim. It is written in the pasuk by giving Maaser of produce. Many peopel extend it to Maaser of money as well though.
Also, how do you know it doesn’t work? The promise doesn’t say that as soon as you give the Maaser, more money will magically appear in your bank account. It could be that it takes time. Have bitachon that Hashem will fulfill the promise and eventually he will.