All of your puzzles pieces should be available to complete the puzzle, but some might be hard to see. When you start a game there are pieces resting on top of the other pieces with similar colors and patterns.

Check the edges of the game area to see if your puzzle piece(s) have been pushed outside the normal view. If you're not able to zoom in/out, open the Toolbar (Press the triangle in the bottom right corner to expand the Toolbar if it is hidden), ensure that the Lock icon is in the "Unlocked" position seen below.

This will allow you to zoom in/out by pinching your fingers together on the screen.

Use the "Scatter Pieces" button and select the "Scatter and Zoom Out" option to clear away stacked pieces and zoom all the way out.

Try using the "Hint" option by tapping the lightbulb icon in the toolbar, this may give you an idea of where the missing piece is on the board.

Lastly, you can also toggle the Show Edge Pieces feature on/off to see if you notice where the missing piece went to.