Why does first thru fifth gears slide in smoothly but reverse wont even shift in on 1998 Honda Accord

The transmission was just put in but it maybe something to do with the clutch i guess. A little bit of noise from shifting but once running for sometime it stops. The gears can be some what stiff but not noticeable.

Asked by for the 1998 Honda Accord
Re-built , reman. , or used trans.? Was anything done with the clutch assembly while trans. out??