isomers ( a single peake but not several peaks)

i'm trying to develop an HPLC method for analysis of some compounds which exist as a mixture of several isomers ( 2 to 4 isomers). I'm using hypersil star column ( C18,150, 4.6, 5u). My work requires that each compound must be eluted as one peak ( regardless of number of isomers it composes of) , however while trying to develop the method , each compound elutes as several close-spaced peaks but not a single peak. can any body help me in this regard?!!  thanking you

fresher:
You have a "selectivity" problem.  In this case, your column is more selective toward each isomer, which is not what you desire.  Most C18 columns will separate by the number of carbons in a molecule.  When two molecules have the same number of carbons (isomers, for example), sometimes their shapes are so different that the retention is different for each shape (more linear, or straight, molecules usually have longer retention times).

You need a column that will not be so sensitive to the shape differences.  One common way is to use "normal phase" chromatography (nonpolar mobile phase, polar stationary phase).  These systems are not as sensitive to shape.  But if all of your molecules are simlar none may be separated.

What kind of isomers are these?  Double bonds?  Aromatic rings?  If we have more information we may be able to suggest a different phase.