help with interpretation of multiple regression results

Dear forum members,

I am running multiple regressions with SPSS and I need help with possible interpretation of my results.

Firm characteristics reveal that focused firms have more profitable growth opportunities (as measured by their mean Tobin's Q of 3.5). In contrast, diversified firms have less profitable growth opportunities (mean Tobin's Q of 2.2). Further, my multiple regressions reveal a positive relationship between Tobin's Q and stock abnormal return (the dependent variable).

However, although focused firms are have higher Tobin's Q and there is a significant positive relationship between Tobin's Q and the dependent variable (abnormal stock return), I do not obtain a significant relationship between the corporate focus proxy and abnormal returns. I am confused and wonder whether there is any plausible explanation for this?

The regression equation is the following:

Abnormal return = a + b1*CorporateFocus + b2*Tobin'sQ + b3*RDIntensity + b4*firm size + b4*Issue Size+b5*stockvolatility + b6*runup

Thanks a lot!