Transmisson gear not shifted to R

The shift lever in revers (R) doesn't mesh in the gear and when driving forward there is delaying from gear 2 to 3 then it goes smoothly. After I check it in workeshop I found no failures or deffects in electronic sensors but the oil quantity was less so I drained it and added new oil but the proplem still there. Could you help me to know the reason of thise problem!