Brian, try using a lighter line (like "parachute" cord) that is strong enough to tug without breaking, and put a weight on the end of it. If that doesn't work, you should be able to buy an electrician's "fish tape" at any hardware
store, ten bucks for 50-75' of thin steel
tape that you can often force up or down the mast, past obstructions.
It could be old insulation
, stuffed in to stop halyard slap, or something harder which might fray your halyard, so if there's any way to figure it out, try outsmarting it rather than just forcing past it.