Topic says it all. I cannot seem to get any lubricant into my linear rails. I’ve tried injecting lithium grease into the holes in the carriage, but without success. I’ve tried spraying white lithium grease under the carriage in the hope it might sneak its way onto the bearings, but no apparent luck. I’ve tried filling the rail grooves with grease; it still sounds like the carriage is packed with gravel.
Any advice? Please? My baby is in pain.
What kind of linear rails?
Are they new?
Particularly if this happened after use, it’s possible you now have bearing balls that have spalled and are no longer round.
Or you might actually have some sort of foreign material in the recirculating channels, and grease will just help hold that foreign material in place.
For most uses of linear rails, light machine oil (e.g. sewing machine oil) is the typical lubricant, rather than grease.
If this were my machine, I would remove the rails and trucks/carriages from the machine, then (carefully, to avoid losing any of the balls, over something that will catch them!) would disassemble them and wash them out well with light oil, then re-assemble them, and hope that whatever was grinding was softer than the bearing steel used for the balls and rails. 
It’s really easy to lose bearing balls when doing this. I have done that, and the carpet in the room with my 3d printers has some bearings embedded in it. 
WD-40 makes a spray silicone that is good. I used to work on automatic doors, and the twin slide model had the industrial style of 12mm rails. This stuff really works for removing the debris and lubing at the same time. But like MJ said if you clean them and they still sound bad, they are probably toasted.
Although they are pricey, in the right situations they are great…