Crying in front of another person, even a T, can be hard. I can remember that with previous T's and in previous therapies I've never cried. I didn't like crying in front of other people.
My current T has seen me cry. I'm not sure if she had seen me cry the first time I was in therapy (for social anxiety). I think I didn't really cried, only some tears.
But this time in therapy, she has seen me really cry a few times. I didn't really want to cry, at least not so bad. I was already feeling terrible and then we started talking about a difficult topic and I just couldn't stop myself from crying.
But there were also session we talked about the same hard things and didn't cried. I was on the verge of crying, but no tears.

I didn't really allowed myself to cry, I just couldn't stop myself from crying. I don't know why sometimes I cry while talking about something and at other times I do cry while talking about something that less affecting me than other things.

I've also felt that I should cry or at least have tears so my T know how much something is affecting me. I feel like if I don't show much emotion while telling something that's really hard for me, then she would think it doesn't really affect me.

You could talk to your T about this.
Why do you try to hide it? It could help if you know why, then maybe it can help you to let the tears come if they come.

And there's nothing wrong with not crying in therapy.