A bit more detail would help!
1. Was it there before you ever cleaned the windows?
2. If it is "oily" then wfp will struggle as you will need detergent to break it down - is it on an upstairs or downstairs window?
3. If it's upstairs try an applicator (with GG4 or even "Fairy") on a pole and scrub a dub dub, blade off and then thoroughly wash and rinse with wfp.
Hope this helps!