It is important to always make sure you clean off the zerk before engaging the tip of the grease gun to it.
About every 3rd or 4th mowing (or sooner if desired), give each spindle/zerk about 3 pumps of grease with a hand powered grease gun.
Grease is the cheapest part to replace on a machine, and the easiest to install.

Mownie, good advice. At the start of the season how much should I put in?
On my big tractor I can see grease coming out when it's full.
Jerry

Mower decks on X500 JDs have naked (without seals on either side) caged ball bearings . There are seals at either end of the spindle so pump till the grease oozes and then stop.

Get you some of these Zerk caps. They're great for keeping Zerks clean.

One of the three oozes the other two the grease backs up and come out around the nozzle. I poke the zerks with a pin to see if the ball was stuck and they both moved. Is too much gunk built-up around the spindles?
Thanks,
Jerry

Does your grease gun have an adjustable collar at the Zerk?

If it does then you might need to seat the grease gun on the Zerk and judiciously tighten the collar for a good connection. Loosen the collar slightly to remove the grease gun.

Make sure you push the collar straight on the zerk and keep it straight, sometimes skewing the angle of the collar to the zerk will cause it to leak at the connection. If the zerk is plugged, which is doubtful, but if it is, there is a tool that you fill with light oil, attach to the zerk and then smack the tool with a hammer, that will usually blow the zerk open.