What do I do if a swimmer misses the touchpad or touches it softly?

Your watch and button times exist for exactly this. Under Article 102.16:

• Stop your watch and push your button when any part of the swimmer touches the wall, whether or not the pad registered.
• Lane timers report a late or missed pad touch to the head lane timer or recorder.
• Head lane timers note on the timing card if the swimmer delayed touching or missed the pad at the finish, or if there's any reason to think the button or watch times are inaccurate.

A missed or soft touch is one of the triggers for the Referee to check for a primary system malfunction (102.23.4C). If one is confirmed, the backup times for that lane are used to set the official time (102.23.4D). The Referee investigates any apparent timing malfunction (102.11.7).