+1 on adjusting the clutch cable. I would also recommend trying to pull right up into second gear from neutral with the engine off while sitting on the bike, maybe rock it back and fourth and see if it feels like it goes in. If adjusting your clutch and doing what I mentioned doesn't work you may have a more serious issue with either your transmission, or linkages.
2008 NINJA 250R TOTALED
2014 NINJA 300 SE SOLD
1992 NINJA GPX ZX600C PARTED OUT
1989 NINJA GPX 600R
2008 NINJA ZZR 600