block invisible on blog and node frontpages

I'm having trouble displaying the twitter block on the node and blog page.
http://jawi-online.nl/node
http://jawi-online.nl/blog
On these pages I can't make the tweetblock visible in any block. On all other pages the block is visible (in the 4th preface bottom column)
Checked my theme, and everythings seems OK
Can this be a module issue for collected pages like /node and /blog?
Thanks for all comments.