I gave a site a makeover about 4 months ago and the daily hits have been gradually rising - between 60 - 150 a day.
The site is showing fairly high in Google for its keywords but it still has a pagerank of 0. Is there any reason why the pagerank could be so low when the site is obviously performing?

What is your website domain age and it will in Google ranking position and most probably search engine will consider the domain age to provide PR and ranking position too and for that be concentrate on your off page optimization work to get back links and PR too.

There are two domains for this site - the .com and the .co.uk.
The .co.uk is the main domain and this is only 6 months old. The .com domain points to the .co.uk domain and this has been registered since 2001.

google pr only updates twice a year, it could take another 6months before your pr increases

That would account for it. I'll try and be patient. The rankings and traffic are fine so it should go up after the next update.
Many thanks folks for your replies.

Page rank depends upon the number of backlinks and the quality of content of you site

Getting PR is not that easy. Traffic is not the only thing that Google considers to give you a high PR. Relevancy, Unique Content and lots of Backlinks are also needed to have it.