Fresco Eyedropper disappears, cant make multi colour brushes

I am on ipad 3.4.0 version. I have different colour overlapping blobs on the layer. I select eye dropper and move the eyedropper to area of interest. The eyedropper disappears and I cant create multicolour brush.

I have found a work around - insstead of moving the eyedropper, I have to move the payer and bring the area under the dropper. This is inconvenient and often inaccurate.

Thanks in advance.