What wallet are you receiving the btc to? Is it an SPV wallet like Electrum?
If so, it's possible that someone might have generated a lot of addresses via the API and the API is now generating addresses out past the gap limit of your wallet... so your wallet is not registering the new transactions as it is not looking for them.
Have you also tried using
the BIP39 mnemonic tool to manually check if the addresses being generated by the API match ones being generated from your xpub? You don't need to input your mnemonic or xprv... you can paste your xpub into the "BIP32 root key" box at the top of the page, click the "BIP32" tab... and set the custom derivation path to simply "m" (or it might be "m/0" depending on which xpub you're actually using)... you can then generate as many addresses as you need to check that the API is actually generating addresses based on your xpub.