Yes, its so common that I would say it is the normal reaction
@Jo-May
Dont try to explain why they cant go home as they are unable to understand the reality of their situation - in their mind there is nothing wrong with them and they were perfectly fine at home. So now is the time to use what is known on here as "love lies" - lies that are told in order to prevent or reduce distress. I have also heard them called "therapeutic untruths" if this is an easier concept for you.
I dont know the circumstances of them moving there, but my mum moved to her care home from hospital, so I told her that she was convalescing and she could go home when the doctor said (which was sort of true, although I knew that this would never happen). Mum was happy with this. Otherwise try things like you have to get the doctors permission and are awaiting his reply, or even that you cant go now because its too cold/wet/dark/whatever to go, or there has been an accident and the roads are closed, or whatever other explanation will pacify your parent. Try using distraction too. "Yes, but we cant go now mum/dad, because it will be dark soon. Would you like some cake? I brought some with me" I always brought a little treat cake, or sweets, to produce when needed as distraction. Other things I used as a distraction is a walk round the garden/courtyard or sorting out her drawers. Watch your body language - I found that mums mood reflected mine, so that if I was upset or anxious about something she would be too. So staple a bright smile on your face and go in with an air of Everything Is Lovely and try and convey that you have no concerns about the place. If they truly get stuck in the "want to go home" loop, be prepared to cut your visit very short. I dont know how often you are visiting, but in the initial stages its best not to visit all that often. This sounds counter intuitive, but they needs to learn the routine of the home and to get to know the staff and other residents, and they wont do that if you are visiting a lot.
Mostly, though, give it time - it usually takes people a couple of months (sometimes more) to settle. Once mum had settled she thrived and was happy there and this is a common reaction, so dont doubt yourself or feel guilty about what is happening to your parent