Why is my air spin hair styler slowed down?

what happened:

My hair dryer-brush— an Infiniti Pro Conair Hot Air Spin Styler— used to spin at a high speed, but it slowed down recently.

what I did:

I opened the device, there was no hair clogged.

But the rotor did not spin fluently. Even it couldn't be spinned manually.

I tried to put some oil near the both ends of the rotor, it was of no help.

I couldn't remove the element barrel,as everything was pressed. Therefor I couldn't expose the rotor from the base.

What I want to ask:

How can I fix it's slowing down?

Thanks