Yamaha 8hp Lost Gear

[KENL]

After I install a new prop shaft, impeller, and etc. Everything seems working except the forward gear, now i have to crank the gas to 1/2 way before the prop will move, but the reverse gear seems working fine and get enough power, only the forward gear lose all the power? I wonder what did I do wrong now? it is the clutch need to be replace as well? please help. Thanks

[boats.net]

Check shift linkage is adjusted properly, not bent or broken. Shifter inside gearcase may be broken or installed wrong.