First check and see if that theme has its own options to change from excerpt to content.
Depending on the theme, but try and find every 'the_excerpt' and change to 'the_content', usually in index.php when I do it..
Maybe try making a copy of the theme on the server that works, rename it, load it to the /themes of the blog that doesn't work and try that.
I also sometimes see people use things like 'the_content_rss'... Don't do that..
__________________
Join the BEST cam affiliate program on the internet!
I've referred over $1.7mil in spending this past year, you should join in.
I make a lot more money in the medical field in a lab now, fuck you guys. Don't ask me to come back, but do join Chaturbate in my sig, it still makes bank without me touching shit for years..