You haven't cross-contaminated between acrylics and enamels have you?
Tamiya actually has both types, ie. acrylics and enamels. They're not available in all countries (eg, not available in the US), but in locations where they are, they're often sold side by side.
The acrylics come in round jars and the enamels in rectangular jars similar to the old Testors range. The thinners are not cross-compatible. Acrylic thinners are labelled X-20A and enamel thinners are labelled X-20 (in the 10ml size it has a clear cap, but larger sizes have a blue cap).