Tab function not working to create multi level list?

Hi everyone,

This is a very silly question but I just started to experience an issue with my word documents. When creating a set of questions, my tab function is not working to go to the next level.

Desired outcome :

1. What is 2 + 2?

a. 4

b. 6

c. Apple

d. Lion

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Issue

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1. What is 2 + 2?
2. 4
3. 6
4. Apple
5. Lion

So after creating question 1, I would press 'enter' then 'tab' for it to create a multi level list, but now it just remains at the subsequent number and give an indent. I tried restarting Word, my computer but still experiencing the issue.