This is what I would do.
1. Remove tension from the front derailleur by releasing the cable.
2. Check the limit screws on the derailleur when getting in the highest gear.
3. Check the cable and cable housing for resistance by sliding cable and housing
4. Check the shifter to make sure the clicks are engaging with no cable load.
5. Simulate cable load with hand and check shifter clicks
Go from there.
if
#2 ,
#3 and
#4 are good then it sounds like the shifter needs a service or replacing.
- james
www.buckyrides.com
https://www.etsy.com/shop/Buckyrides