In order for each times table to be 'mastered' your child needs to answer all of the questions in the 'Learn' section for each times table. This can take up to 10 times of doing 'Learn' for each times table, depending on ability and accuracy. Your child also needs to get 15/15 correct in the last learn that they do for each time table.
Once the 'Learn' has been completed The 'Challenge' will then become unlocked and that needs to be completed a minimum of three times with 100% at least once.
Once all times tables have been mastered, they will all unlock and your child can then access any of these again to ensure they are practicing and retaining their knowledge.
It is also worth checking that the most up to date version of the DoodleTables app is being used.