Tinkers: 1.12-2.13.0.171
Mantle: 1.12-1.3.3.55
T. Tool Leveling: 1.12.2-1.1.0

For some reason my bow is only gaining 1 XP per kill the bow is made of Plastic, Dragon Bones, Regular String, and paper, the bow + arrow is a hit kill weapon to a standard 20 hp character but for some reason it only gain 1 xp per kill, to test this i made a full Dragon Bone bow witch is gaining 13~14 XP per kill but has a much longer draw speed.

The first image is the "fast" bow that only gain 1xp
The second is the slow bow that gain regular XP

Yes, bows get xp for shooting things, but not the damage dealt (the arrows get that). To balance out bows they get xp relative to their drawspeed.

Is there a way for me to add more xp gain? Or tweaking the mod script will be necessary ?

Having it fire 1 arrow per second is probably optimal for leveling.. other than that you can always change the configs for it to require less xp.