Tight Seatpost Installation
Never use a hammer.

In order to check the inside of the frame, visual inspection most times FAIL, because the problem is caused by tiny bumps or misalignments that are not easily seen with naked eye and even felt with fingertips.

I would recommend that you take another seatpost (an old one you can take/borrow on a local bike shop) and try to insert more aggressively. You should polish the seatpost first someway (steel wool, for example).

You force it inside a bit, then take it out. Probably, there would be some marks on the seatpost surface, suggesting if there is some irregularity inside the frame. Most common are bending around the vertical cut around the clamping nut, or deformation caused by welding the tubes around that region too deep (to the point of almost melting the inside of the tube.

The seatpost tube of the frame might be not-straight, which would be a manufacturing defect.

In the end, you could polish/sand/grind inside the frame to make the seatpost go down easier, but since seatposts that don't go down so easy come out very hard afterwards, and your post has a carbon surface, taking the frame to the bike shop to check if it needs replacement or special service (with machining tools to improve surface quality and alignment) could be necessary.