Post level 20, your rank is determined by the amount of "light" you have on your gear. You normal xp continues to accumulate towards Motes of light. You can view that by hovering over "20" in your options menu.
What you need is to get armor pieces with more light, AND upgrade that armor. Each "Defense Upgrade" provides +2 light. So you'll want to start upgrading your gear and you'll notice that you'll increase in rank.
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oh, so i've probably maxed out whatever 76 light gives me then? and when i go higher than 76 it will then resume increasing?
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Correct. You've maxed out your armor at 76 light. You need to get better gear that holds a higher capacity and upgrade that gear and your level will increase.