I was born in DC with diplomatic parents than moved to Europe.
I’ve return to the US about 2years through a B1/B2 visa and was able to obtain a SSN.
Since then I’ve considered myself as a lawful citizen. I’ve been working, have voted and performed jury service. I recently applied for a US passport to visit an ill relative and was denied due to diplomatic immunity at the time of birth, a 14th amendment exemption.
I'm really perplexed and would be appreciate any advice.
Thanks

The 14th amendment, sometimes called the citizenship clause precludes automatic US birthright citizenship for children born in the US to diplomats that have diplomatic immunity. This is because under the 14th amendment, your parents were not "under the jursidiction" of the US for certain laws.

The FAM is the definitive answer to your question:

7 FAM 1116.2-2 Officers and Employees of Foreign Embassies and Consulates and their Families(TL:CON-64; 11-30-95)a. Under international law, diplomatic agents are immune from the criminal jurisdiction of the receiving state. Diplomatic agents are also immune, with limited exception, from the civil and administrative jurisdiction of the state. The immunities of diplomatic agents extend to the members of their family forming part of their household. For this reason children born in the United States to diplomats to the United States are not subject to U.S. jurisdiction and do not acquire U.S. citizenship under the 14th Amendment or the laws derived from it.
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b. The names of diplomatic agents accredited or notified to the United States and who have full diplomatic privileges and immunities are published every three months in the Department's Diplomatic List, often called the “Blue List.” The Diplomatic List also gives the name of the spouses residing with them, but does not include other members of the family forming part of the household, although they may be entitled to privileges and immunities.

You need to find an experienced nationality lawyer to assist, see if you can find a certified specialist.

If this was helpful, rate this answer! Good luck.

The above response includes the direct cite from the DOS Foreign Affairs Manual (FAM). This is where your research should start. Good luck!