I've done a lot of successful iphone 5 and 5s repairs but I've never heard of using silver conductive paste with the repair. Not unless you were building the screen yourself before installing it.
Is the polorizer on the front of the display or are you installing this inside of the device under the replacement lcd/digitizer?
Many things could affect the touch. From a bit of static bouncing from your finger to the digitizer connector. Or touching the three bits of solder that are on the flex cable of the new screen. Or you could be looking at a simply defective part.
Hope this helps.
Good luck.