With 6500 hrs in 707, and 2000 in 727 (same gear system) never had that problem.
Are the gear lights showing any greens/red...?
Many moons have passed since I flew them, but would try to move lever UP/DN.
If that does not work, pull the trigger while doing same UP/DN.
Also be sure hydraulic pressure is OFF (zero psi) then extend gear manually.
Then again try to re-position lever to DN...
Used to teach 707 classroom/simulator... Never had that situation.
Curious if anyone comes with an idea...
xxx
Happy contrails