Riding lawn mowers can sometimes be tricky to get into gear or out of gear. These machines are made on a high speed production line and are not always exactly perfect.
Sometimes the shifting rods will become slightly bound up and a gentle rocking back and forth will get the mower into gear. You can also check to see that grass is not interfering with the shift linkages. Rocks and caked on dirt can also cause the shift linkage to bind.
Clean out the area with a leaf blower or compressed air and then lubricate the shifting rods with dry silicone or similar lubricants that don’t attract dirt and debris.
If this does not fix the problem, then something internally may be the problem or you may not have enough fluid in the transmission. Check the fluid level and add more if necessary.
If the problem is internally in the transmission or gear case, it would be best to have a professional resolve that for you. You can find a lawn mower transmission repair shop on our Locations page.