Insert failure in unordered_map

I have an unordered map defined as (within an ordered map):

std::map<ClassId, std::unordered_map <KeySPtr, ObjectSPtr,KeyClassKeyDataHasher, KeyClassEquals>> collection_;

The hash function is basically fnv:

// The FNV hash function
static inline size_t
fnvHash (const void *data_ptr, int len)
{
    uint8_t  *ptr = (uint8_t *)data_ptr;
    uint32_t hv = FNV_32BIT_OFFSET_BASIS;
    int     i;

    cout << "data_ptr: " << data_ptr << " length: " << len << endl;

    for (i = 0; i < len; i++) {
        hv = (hv * FNV_32BIT_PRIME) ^ ptr[i];
    }

    return hv % 0xffffffff;
}

The insert is done as follows:

std::pair<
            std::unordered_map<KeySPtr,ObjectSPtr,KeyClassKeyDataHasher, KeyClassEquals>::iterator,
            bool> ret;

ret = collection_[class_id].insert(
            std::pair<KeySPtr, ObjectSPtr>(key, object));

What I'm seeing is, for some cases even though the generated hash is unique, the insert fails. And when I check the return iterator, it has some keySPtr, which was inserted with a totally different hash.

What am I missing here? Is it something to do with the 32-bit hash that is being generated, and it is not working with my unordered map hs table

Thanks!