I bought robux and it didn't arrive in my account

You would need to talk to the roblox publisher. Here is a link to there help page that deals with your situation.

Roblox help page

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today Esaiah

Sorry to hear that mate, if you double check your Microsoft account order history, the transaction appears there or not yet?
https://account.microsoft.com/billing/orders

If yes, appears as approved?

Please let me know, I'll be waiting for your reply
Greetings from another Microsoft user

Hello,

My son just redeemed his $15 Microsoft gift card and bought 1200 robux money but it is not showing up in his account. It's sayin he still only has 3 robux. Any ideas what is going on? I got the email that the card was redeemed so it seems to have taken the
money but it's just not showing up in his robux account....

Thanks for the answer.

The last one hasn't got a "Tax Invoice" yet, but they are both drawn from my bank account. They are both marked as completed with green letters.

The first one was a very small amount, and I bought it "in game" for a specific item. I waited a few days, and today I just bought a pack of Robux from the main menu in the Robux app, because that's how I usually do it. But the same thing happened.

Oh thanks alot Jesse E and also if I use one microsoft account to get robux on my roblox account and then use another microsoft account to get robux on the same roblox account what happens?