You have a defective touch screen panel, due either to a defect in manufacturing or to improper installation.
You will have to re-open the iPad to correct the mistake (if it can be corrected), or to replace the front panel.
CAUSE 1: touch-screen flat cable pinched (improper installation) or broken (too much pulling) during installation:
FIRST, before re-opening the iPad/ Try pushing on the touch panel ~1 inch above the bottom left corner (you can see the exact location in step 27 of this guide). If the iPad starts showing ghost typing (things move without you touching them), it means you have pinched the cable between the glass panel and the aluminum frame.
SECOND, after re-opening the iPad/ If you do have ghost typing, try unfolding (un-pinching) the touch-screen flat cable. Test the front panel, if you still have non-responsive areas, you will have to change the front panel. If, however, the issue is fixed, then try re-installing the front panel following advice ipad glue/adhesive doesn't stick properly to avoid pinching the cable again. But I am skeptical that it will be fixed because often the damage is irreversible.
CAUSE 2: A/the touch-screen flat cable has not been properly re-inserted or B/the retaining flaps not properly flipped back in place or C/the retaining flaps have been displaced or D/the pins of the connector have been damaged, Step 32 of the same guide.
A/ and B/ You just have a loose connection. Reseat the cable and your problem should be solved.
C/If the retaining flaps are loose but not broken, you can put them back in place carefully, using a magnifying glass and some sharp tweezers. Good luck!
D/ If the retaining flaps or the underlying pins are broken, you will in my opinion need the assistance of a specialized tech to fix them.
CAUSE 3: the flat cable is not pinched, it is properly seated in the connector, and the latter is in perfect condition: then you have a defective touch-screen panel that needs replacing.
My advice is to connect the new touch-screen panel, and before re-gluing it to the aluminum frame, test the whole surface for defects: press on one of the icons until it wiggles, and drag it all over the surface. If it consistently drops along one of the horizontal (or vertical?) lines or around a certain area, you will know this area is a dead area and the panel is defective.