And please do a reasonable number of tests of your code before posting. This is a truncated version of the function to point out the error:
def isprime(n):
for ctr in range(2,n): ## do not use 'i', 'l', 'O' as they can look like numbers
if n%ctr==0:
return 'Not a prime number',ctr,'is a factor'
else:
return 'Prime number'
print isprime(9) ## 9 is not prime-->3**2