Without knowing anything about your indoor envirenment, we can't help you.

The usual issues are lack of sufficient light, poor quality growing medium, improper watering.

Perhaps some pictures would help.

I'm sorry, but garden soil can turn into a very undesirable growing medium when used in a container. And sitting a plant on a sunny window ledge can never be compared to an outdoor, full sun environment.

The sun, filtered by glass, beams in from one direction only. Your plant sits with its back facing a dim room and with a ceiling overhead.

Also, never put pebbles in the bottom of the pot under the soil. Mary meant for you to raise the whole pot up with something so that nothing impedes the evacuation of excess water. It could even be three coins placed so that the rim of the pot rests on the coins rather than flat on the plate.

Thanks for all of that information. So would my best bet be to repot them in to some more suitable potting mix and place them outside with a better drainage system?

Given that it is winter at the moment I'm wary of keeping them outside constantly or actually planting them in the bed of my vegetable garden.

Repotting couldn't hurt, might help. Outside during the day as long as the temperature is warm enough (at least 50F/10C). Just make sure to gradually introduce them to sunlight so they don't get shocked by too much too soon. Or you could get an inexpensive florescent light or even a grow light and use that to supplement the window light.