I'm hesitant to tell you to yank harder, not knowing exactly what you have. Can you take a look at the pictures in the manuals below and see if you can confirm that you have a 70C, 71C, or 72C? Which one of the three doesn't exactly matter for your specific question.
If put the 70C service manual here for you:
And, I have the 71C/72C owner's manual, service manual, and a couple of spec sheets here (I have a 72Cs in my present boat):
Depending upon what year your transmission was made (not to my knowledge model), it might have a little T handle on the dipstick, or it might just be a knurled knob.
In either case there is a threaded rod that connects to the cap or handle and goes down and screws in at the bottom (not top). If you just have a cap, the cap is attached to this threaded rod. If you have a T handle, the T handle is attached to this threaded rod, but passes through a cap. The cap might or might not move with the handle -- but the threads will.
In either case, directly under the cap, at the top of the threaded rod, there is a rubber plug-like seal. The threads can sometimes spin within this seal, but sometimes it spins with them. This seal can sometimes hold the dipstick down even once the threads are loosened. So, once you feel sure the threads are loosened, just pull and wiggle the dipstick out. It shouldn't take crazy strength, but maybe a bit of a wiggle.
Sometimes, when putting the dipstick back in, once needs to push down a bit against this seal to compress it to get the threads far enough down to grab.
There is a good picture of it on page 62 (pdf page 61) of the 70C service manual. There is also some text and a blurry picture of it on page 45 (pdf page 44).
You'll find a couple more good picture of it on pages 5-6 (pdf pages 7-8) of the 71C/72C Owners Manual. (But, I like the picture in the 70C manual better)
The trick is to observe that the threads are intended to hold the dipstick in place from up-down movement, and side-to-side movement at the bottom -- but the plug/seal holds it in place against side-to-side movement at top. But, if this plug/seal holds tight, it can make vertical movement hard, too.
So, once you are sure you have the threads loosened, just pull or wiggle and pull.
Hope this helps!