Your Adventure Rank is capped at certain milestones until you complete a specific Ascension Quest. This quest is required to raise your World Level and continue your rank progression.
Any Adventure EXP you earn while capped is stored and will be applied automatically after you successfully complete the quest.
Troubleshooting Steps
Situation 1: At Adventure Rank 45
Check your Quest Menu for the "Adventure Rank Ascension 3" quest and complete "Ascend: Clear The Ruins".
Situation 2: At Adventure Rank 50
Check your Quest Menu for the "Adventure Rank Ascension 4" quest and complete "Ascend: Clear The Ruins".