Sabrina,
Potting soils aren't what they used to be. I don't know what you have access to there, but it doesn't sound good. You may have better luck by using a bagged mix that is specifically designed for sprouting seeds. These are typically made with peat and perlite, and you'll need to supplement them with a complete liquid fertilizer. There are a few boutique potting soils available here which are made from traditional ingredients, (aged ground bark, peat, humus, sand, topsoil, compost) but for the most part, potting soils have become part of the yard waste recycling chain, and are liable to contain anything. For me, the biggest problem is that they are based on raw wood chips (instead of bark), and the wood starts to mold right in the pot once supplied with moisture and nutrients. These mixes are often dyed in order to appear to be well-aged, and after a few waterings, they return to their original tan color. The industry has few regulations or standards. That I am aware of.
A few years ago, I heard of a widespread problem where a herbicide commonly used in cattle feed yards was making its way into potting soils via the steer manure. Even though it was present in very low concentrations, it was acting as a pre-emergent, and pepper and tomato seeds sprouted in it would whither and die.
If you're stuck using bagged potting soil, sift it through a 4-5 mm screen, pasteurize, and add 25% perlite. Otherwise, you might do fairly well by planting the seeds directly in the garden after the weather cools. At least there, the good & bad soil organisms have a way to balance each other, and with a bit of wire screen to keep the kitties from digging, you might be pleasantly surprised at the results.
