Lights not flashing anymore

I have a Bally Motordome that I had a few bulbs out on, so I deceided to replace all of the 555's under the playing field with LEDs. Most of them worked fine, but I have a few that instead of flashing while in attract mode (and in the game) stay solid and not as bright. I have tried the bulbs in other sockets and they work fine, so wondering what I have done that they are not working correctly.

Ideas?

Thanks!