My heroes stop gaining xp after level 13?

My heroes stop gaining xp after level 13? - Unrecognizable young woman demonstrating prohibition sign showing gesture of refuse using hand and note with sign stop

Why does this happen, And how can I fix my heroes to get more exp?

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My heroes stop gaining xp after level 13? - Woman with paper with cross sign

My heroes stop gaining xp after level 13? - Aggressive black man grabbing offended ethnic woman

My heroes stop gaining xp after level 13? - Woman With Beaded Necklace Covering Her Mouth With Her Hand

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Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Anete Lusina, Anete Lusina, Keira Burton, Mikael Blomkvist