Is it hard for US citizen's parents to get B2 visa?

[zhang]

I recently became US citizen. My parents have visited US many times before as B2 visa holders. They would like to come this summer again. I heard that it's difficult to get B2 visa for US citizen's parents? Is it true? Any advise on how to get B2 visa? Thanks a lot in advance.

[immihelp]

Yes, it can be difficult because once they arrive in the US, you can file their green card.