[R] NA in lm()

[vinod gullu]

Hi all,
I am trying to fit full quadratic model i.e liear +
square + interaction terms to my data (216 data
points), so in my case coefficients are 28 (6
parameters). When i look into summary lm() of model
all cross terms coefficients are NA. What does this
mean? Is the regression is not able to solve for these
coefficients due to some reason.  Or does this mean
that i should not consider these terms.
Regards,