In reply to jhw: Agree with the suggestions re condensation.
If you have a couple of people in the tent, then that's at least 2 litres of water vapour, much of which will condense as soon as it hits something cold - normally the inner skin (particularly if it's solid nylon, not mesh) or your ground sheet.
Your cheaper tent probably didn't have this problem because it was more "breathable", and although it would allow more water vapour to escape, it would not have been as protective in respect of keeping bad weather out as your mountain tent (which is the key objective of the mountain tent).
Ventiliation is the best cure - try leaving a slight gap at the top of each door, both inners and outer (if you have more than one entrance!) to allow airflow to remove the water vapour. If it's a mountain tent, then it will be designed in a way to allow you to open the doors to allow airflow, without reducing the ability of the tent to keep out the elements.
I do a lot of winter camping, and a good gauge of how well your sleeping bag insulates you is if you wake up with a slightly damp (or frozen!)layer on you sleeping bag outer - meaning that it insulated you so well that the outer part of the sleeping bag remained cold enough for water vapour to condense on it.
I'll often take a very lightweight sheet on multiday wild camps in winter, which I lay on top of the sleeping mats. This often helps to keep the sleeping bags and mats a bit drier, which is particularly important if you have no weather/opportunity to dry them out (i.e. cold, wet trips where you are having to pack and unpack damp kit).