It may be that you've fertilized, treated for weeds or otherwise applied chemicals to your lawn that has depleted or destroyed the worm population. You can try buying earthworms from suppliers. At least here in the U.S., there are several companies that sell them.
Again, here, the only types of earthworms that leave ridges and castings on or near the surface are very large varieties that we call "Night Crawlers". These average 5 to 7 inches in length, whereas the garden variety (no pun intended) are only about 3 to maybe 4 inches in length. The small worms do not leave the ridges and castings. I'd order 100 worms per 10 square feet to start. I'd try to "seed' them after a good rain but when the soil has begun to dry. Do it in the evening to preclude bird predation overnight... gives 'em time to burrow...