Why will it only go half way down

There are many different types of garage door openers -- there is probably

A device that detects when door is closed -- can not tell you what to do as

I have no way of knowing what your opener even looks like??

Could be the stop on the chain, could be the adjustment screw is out of adjustment, could be a track and roller issue, etc...

Tell us more. Is this something that just started with an existing door opener, is this a new opener, a little more detail please.

Does the light start blinking on and off when the door stops or does the door reverse? Chain or screw drive, what brand and model please.