Problems after filling

At a dental checkup a few weeks ago the dentist told me a piece of tooth had chipped off a large bottom molar (which already had a filling in it) and there was the beginning of a decay. Tooth was xrayed and I was told that I would need a new filling and the decayed matter excised. Note I was in no pain etc.

I got a new white filling 3 weeks ago. It was very sensitive after but I told that was because it was a big filling etc. But a week later I still couldn't chew on it. Not exactly agonising pain just hugely sensitive - hard to describe the sensation.

I didn't want to cause a fuss but I called the dentist. Was told he was on leave so I booked an apt for his return (monday last week). He took xrays again and said there didn't appear to be nerve involvement but said the issues were caused by one of the following:
* That tooth can't take modern composite fillings because of it's structure and the way composites stick to the side of the hole which is putting pressure on the root (I have several other composite fillings in other teeth without issue). In this case I need to have the tooth refilled with a traditional black filling - at my cost.
But even this may not work because he said this could be an issue also:
* By no fault of the dentist the root is now dead and I will need to go to another specialist dentist for a root canal - at my huge cost.

Can anyone advise on that first point - can some teeth just not take a composite filling? If that's the case then I'm going to have eventually paid out almost €300 filling a tooth - and then I may just have to go to get a root canal anyway?!?!?!