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:
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.