The reaction is an example of electrophilic substitution reaction. And as usual with all electrophilic substitution reaction these take place easily with aromatic rings with high electron densities. For a detailed discussion, one may refer to any book dealing with reaction mechanisms. A basic book like Organic Chemistry by Morrison and Boyd is very useful. You may also refer to "Named Organic Reactions 2nd Edition” by Thomas Laue and Andreas Plagens 2005 John Wiley & Sons Ltd, Page 280

Dear Dhiv, very simple, U cannot do elctrophilic substitution on a molecule having low electron density on desired carbon. better u try microwave synthesis of same molecule instead of conventional method.

read: IJPCBS 2012, 3(1), 25-43

and

J.Ind chem soc 85(2008), 9, 959

I do agree with the above comments. You have to understand the reaction mechanism step by step. Please see

http://www.name-reaction.com/vilsmeier-haack-reaction

Good luck