collect

Hi, I don't understand some answer about the method collect(). If it is able to collect with 'sin($1)' the polynomial 'a' why not the polynomials 'b' or 'c' with '2^$1' or '$1^n'?

a=d*p*sin(x)+p*sin(x)+q*sin(y)+q*d*sin(y)+sin(y)+d*sin(y)+q*sin(x); q*sin(y)+sin(x)*p+d*sin(y)+sin(y)+d*q*sin(y)+sin(x)*q+sin(x)*d*p collect(a,find(a,sin($1))); sin(y)*(1+d+q+d*q)+sin(x)*(q+p+d*p)

b=q*2^y+2^x*p+d*2^y+2^y+d*q*2^y+2^x*q+2^x*d*p; q*2^y+d*2^y+2^x*d*p+2^x*p+2^y+d*q*2^y+2^x*q collect(b,find(b,2^($1))); q*2^y+d*2^y+2^x*d*p+2^x*p+2^y+d*q*2^y+2^x*q

c=n*2^n+n-3+3^n-n^2*2^n; -3+2^n*n-2^n*n^2+3^n+n collect(c,find(c,$1^n)); -3+2^n*n-2^n*n^2+3^n+n

Thanks in advance, Tatiana Zolo