All the annotation tools are designed to deactivate after one use. If you want an annotation tool to remain on for multiple uses you have three options:
(1) You can double-tap on the annotation tool button. This will keep the annotation tool selected until you tap the button again or select a different annotation tool.
(2) You can tap on the pin button on the tool palette. This will keep the annotation tool selected until you tap the pin button again or select a different annotation tool.
(3) If you always want the annotation tools to remain selected when being used you can go to the Settings in the Case Selection screen, and under User Preferences turn on the Lock Annotation Tools option.