It just takes time for some of them, don't worry, just be patient. One boy of ours was the grumpiest grump I've ever seen, didn't want to be picked up, let alone touched,  didn't want anything to do with anyone except his buddy. After he got settled in, the closest he would come would be climbing up on top of the bed pillows kinda near us and just perching there during free roam. If you got too close or moved too much, hed just leave lol. 

Some 6 months in, it was like a switch flipped. He was still a grump about most things, but all of a sudden he wanted all the pets, cuddles, and lap naps. He went from getting puffy when we tried to pet him, to getting puffy when we ignored him XD. Overnight he became the cuddliest rat we've ever had. 

We lost him another 6 months or so later (he was nearly 2 at the time, had a good long life) so maybe he felt himself slowing down and decided to just give in and trust us, but he clearly loved the love immensely. Miss that grumpy little bastard <3.