Shimano Nexus 7 gear hub slips
If I understand correctly you are trying to use a conventional 7 speed derailleur shifter on a Nexus hub. The Nexus hub does not use a linear cable pull ratio. The amount of cable pull required to shift is not the same from 1st gear to 2nd as required from 2nd to 3rd. A conventional 7 speed shifter pulls 2.9mm of cable between each gear. So you need to get a Nexus compatible shifter or risk hub damage due to improper adjustment.