In order for taxi diagrams to show automatically, you will need the following:

1.  You will need a "Pro" level subscription.  Basic plans do not offer geo-referenced taxi diagrams that are needed for the auto-show feature.

2.  If you have a Pro level plan, make sure you are signed into the ForeFlight Mobile app under MORE > ACCOUNTS.

3.  Under MORE > SETTINGS, make sure the switch is turned ON next to AUTO-SHOW TAXI.

4.  The airport diagram has to be geo-referenced in order to be displayable.  Please see the article:  

     How can I tell if a plate or diagram has geo-referencing?

5.  Once you have verified the prior 4 items, you will need to accelerate above 60 knots GPS ground speed for a minimum of 15 seconds and then slow below 40 knots GPS ground speed upon landing within the runway environment and near the airport altitude.