I have code using the Crypto++ library where I am encrypting some bytes, I am having a problem where some bytes are not being encrypted.

This problem only occurs when I use a key and an iv for each byte

If I use a key and a single iv this problem does not occur

I have following code:

std::vector<uint8_t> sizeTToBytes(size_t value) {
    std::vector<uint8_t> bytes(sizeof(size_t));
    std::memcpy(bytes.data(), &value, sizeof(size_t));
    return bytes;
}

std::vector<uint8_t> SHA1Hash(size_t value) {
    std::vector<uint8_t> bytes = sizeTToBytes(value);

    std::vector<uint8_t> hash(CryptoPP::SHA1::DIGESTSIZE);

    CryptoPP::SHA1().CalculateDigest(hash.data(), bytes.data(), bytes.size());

    std::vector<uint8_t> truncatedHash(hash.begin(), hash.begin() + 16);

    return truncatedHash;
}

CryptoPP::SecByteBlock GenerateKey(const std::vector<uint8_t>& password, const std::vector<uint8_t>& salt, size_t iterations, size_t keyLength) {
    CryptoPP::SecByteBlock Key(keyLength);

    CryptoPP::PKCS5_PBKDF2_HMAC<CryptoPP::SHA1> pbkdf2;
    pbkdf2.DeriveKey(
        Key.data(), Key.size(),
        0, // purpose identifier
        password.data(), password.size(),
        salt.data(), salt.size(),
        iterations
    );
    
    return Key;
}

void BytesEncrypt(uint8_t *my_byte, uint32_t byte_len, size_t unique_id, size_t unique_val, size_t unique_val2) {
    auto NewIV = SHA1Hash(unique_id);
    auto NewSalt = SHA1Hash(unique_val);
    auto NewKey = SHA1Hash(unique_val2);

    CryptoPP::SecByteBlock my_key = GenerateKey(NewKey, 213, NewSalt, 32);
    CryptoPP::SecByteBlock my_iv = GenerateKey(NewIV, 137, NewSalt, 12);

    CryptoPP::GCM<CryptoPP::AES>::Encryption e;
    e.SetKeyWithIV(my_key.data(), my_key.size(), my_iv.data(), my_iv.size());
    e.ProcessData(my_byte, my_byte, byte_len);
}

Now if I change the code to use a single key and iv it encrypts all bytes

Can anyone tell me how I can fix the problem?