Apparently she's too shy to tell him; she may be afraid of rejection, which I could sympathize with. But like Millefeui told Serena a few episodes ago; she needs to make a move for Ash's affection soon or someone else will.
That's indicating that there may be something done, and Serena will actually ask Ash out. I wonder how it will be like. I think it's likely to happen now that Millefeul said this.