Transmission problem

All of a sudden my automatic 3 speed transmission is not shifting properly.
My 71 Mach 1 came with a 302 2V and I am not sure but may be the original transmission.
The car when I bought it has a 351W and had a 4bbl put in. I have owned approximately 5 yrs with no major problems.
I only drive occasionally, few Times a month and do not drive it hard.
Yesterday I drove it about 6 miles no problems let it set for an hour then drove it back about 5 miles stopped went into store for maybe 10 mins no problems.
Started up the car, put in drive took off. Stopped at a light and when taking off seemed to be in neutral. Shifted into neutral and back into drive. It took off but didn’t seem to shift to 1st or 2nd. It will take off in 1st and I can manually shift to 2nd but not sure if it shifts. It will go in reverse.
I checked the fluid when I got it home in park with engine running. It seemed to be full. Not sure if transmission is C4 or 6.
I am going to take it out again today and pay more attention to the shifting.
Any ideas

Thanks everyone. I took my car out this morning and it did not shift at all. I basically had reverse and 1st gear.
Someone on another forum said it might be the vacuum modulator or a different vacuum hose cracked or off.
My friend agreed that it might be a vacuum hose. He came over We took off the air filter cover. He shined his flashlight behind the carburetor and said there it is.
Rubber hose was disconnected. The only thing we thought was the hose was old and gradually vibrated off. Hooked it up, car started right up and sounded good. I am going to get new rubber hose.
Took it for a ride, it ran and shifted very good.