Position the A and B markers and then tap the "Add Loop Mark Button" to save a loop mark. The "Add Loop Mark Button" turns into a "Delete Loop Mark Button."
Next, touch the yellow A handle in the top or bottom waveform, and move it to the left or right. Once you do that, the "Delete Loop Mark Button" should change back into an "Add Loop Mark Button." You can now tap it again to save another loop.
The key thing to understand is that loop saving is done regardless of where the playhead is. In other words, the playhead position doesn't really matter. Loop saving is based solely on where the A and B positions are.
Note: The A button (next to the X and B buttons) will disable when the playhead is in a position where it would not be possible to move A given where B is located. The two cannot wind up on the opposite sides of each other (A cannot be after B, and B cannot be before A).