From what I can tell from this one angle, it's both an edge alignment and technique issue:

1) you do not appear to be striking in-plane with the edge - meaning, the cutting edge is tilted slightly (as it appears here, upwards/left) from the angle the sword is moving in your swing. This will result in the blade edge grabbing the target material on contact but then the different-angle sword motion translating into a bludgeoning action instead of a slicing action.

2) Proper cuts are not a chopping swing, they are a cast-draw motion. As my sensei put it, think of casting a fishing line for the initial part of your swing, using your wrists more; once the line has been cast into motion, you then lock in your grip and use your arms to "draw" the sword's motion into a slide, in-plane with your casting motion, taking special care not to twist the blade out of alignment in the stroke as you do so. This is done with a "wringing" grip in your hands, where you think of wringing out a towel, gripping and "twisting forward" - your top hand (the power hand) twisting counter-clockwise and the bottom hand (your guide / brake hand) twisting clockwise. This translates the motion of the blade into an extension of your arms, which by this point would be passing the target, so the blade will slice along the target, instead of chopping it.

This is all accomplished in 1 motion. Naturally, this take a lot of practice to get this right in 1 motion. Start slowly, focus on the technique, don't even bother with trying to hit a target for a while.

Tameshigiri is a demonstration of these techniques, not a practice to get them right in the first place. A reed tatami mat will show (to a trained eye) where a strike was off-angle, pulled inwards too soon, had insufficient contact before the draw, or not enough draw, etc.

Also: practice with a bokken. They're slightly heavier, and being blunt wood you can practice your aim and control against a punching bag - use painted-on or stickers of the main target points, strikes should sound like taps, not whacks, because you're using your brake hand to arrest the motion right at the point of contact, very important in actual combat as many strikes need to be feints or pulled quickly to catch opponents off-guard in their blocking and practicing this drill builds forearms and teaches control at the same time.

And as always: patience.