tamiya clear red hardened in bottle

I have used Tamiya paints a lot for a long time and have never experienced this.  However, only in the last year have I used any of the clear shades.  I last used the clear red around the start of this year.  I opened the bottle to use it tonight and it was almost solid.  I put progressive amounts of Mr Levelling Thinner in it and it came back to something that seemed like Tamiya clear.  Are these paints prone to hardening?