Question: Unable to compute coefficient

Hi, I am trying to compute the coefficient of a polynomial as follows:

a:=(1/11520)*(4518-4320*r^(2*n)-5760*_C1*n^4*r^2-97920*_C1*n^2*r^2-97920*_C1*n*r^2-40320*_C1*n^3*r^2+2020*r^6*n^3+1500*r^6*n^2-1440*r^(2*n+6)+60*r^8-2880*r^(2+2*n)-720*r^4-100*n^5-339*n^2+3018*r^2+4560*r^(2+2*n)*n-240*r^(2*n+4)*n^2-480*r^(2+2*n)*n^4+50*r^8*n-130*r^8*n^4-270*r^8*n^3-170*r^8*n^2-20*r^8*n^5+480*r^(2*n+6)*n+480*r^(2*n+6)*n^3-480*r^(2*n+6)*n^2-1680*r^(2+2*n)*n^3+160*n^5*r^6-102*n*r+320*r^2*n^5+1783*r^2*n^4-34560*_C1*r^2-360*r^4*n^5-102*n^2*r-42*n^3*r-6*n^4*r+551*r^2*n^2+2551*r^2*n-1440*n^2*r^4-3240*r^4*n+240*r^(2*n)*n^2+1200*r^(2*n)*n^3-5040*r^(2*n)*n+240*r^(2*n)*n^4+480*r^(2+2*n)*n^2+97920*_C1*n^2+97920*_C1*n+40320*_C1*n^3+5760*_C1*n^4+2140*n*r^6+2481*r^2*n^3+240*r^(2*n+4)*n^4-3600*r^4*n^3-2160*r^4*n^4+1020*r^6*n^4+1800*r^6-507*n^4-589*n^3+34560*_C1-36*r-1399*n)/((17*n^2+17*n+6+7*n^3+n^4)*r);

coeff(a,r^(-1));

However, Maple complains that it cannot compute the coefficient, which is clearly not zero. Does anyone have any suggestions? I wonder under what circumstances would Maple decide that it cannot compute the coefficient of a polynomial? (I tried a simpler expression involving r^(-1) and it worked.) It is utterly important that Maple picked up the coefficient in my algorithm. Thank you for your insight.