British Drama Toolkit and Mastering Issue?

I recorded a neo classic kind of track with piano as main, some strings and a couple of woodwinds and flutes. I created the piece with the British drama toolki), mixed it as best I could and to what I think sounds nice and sent it to an experienced (?) mastering engineer.

I had created the first version with Studio One in 32 bit and after a lot of back and forth, sending the track with and without any EQ or effects, sending the stems in groups, he asked me to send it in 64bit.

So I upgraded the track and settled over into Cubase as per his recommendation, and after a long weekend of figuring it all out and adjusting the tracks, mixing it again with more depth for the instruments, I finally sent him the 64bit stereo track.

After his initial helpful approach and eagerness to help me I got a relatively short email back saying that some aspects were indeed better while others unchanged if not worse. He thinks it might be down to the libraries. He then said stem mastering could possibly cover the issues but the extra effort and cost wouldn't be in relation to the quality of the end product. Therefore he couldn't help me and master my track.

As you can imagine, I am feeling a bit left in the lurch now. I asked him for more details. But currently I am really stumped as to what to do. I had high hopes for the track, that's why I wanted it professionally mastered, and I wanted to release it properly. (Not to add that I had to pay him upfront - a quite common practice in some parts of Europe).

But I am now left wondering what "could be wrong" with the libraries? Afterall they are recorded under great conditions with professional musicians and probably eqed and mastered etc. Better than what I could get if I hired a string quartet and a local recording studio (with probably inexperienced audio engineers for that kind of music...).

It also leads me to the questions: how much EQ and processing do these libraries require? To my ears they already sound pretty good any cleaned up, too.

I would really appreciate your advice and maybe you could even recommend a Mastering Studio or Engineer for neoclassical music.