Potting mix just isn't designed for optimal seed growing, so it isn't surprising you had limited success with it. Any brand would have the same problems. Seed mixes are usually formulated to be low in nutrients, fine textured and quick draining. Potting mixes are heavier and contain more fertiliser.

Another variable could be temperature. Are you sure the temperature was the same this year as last?

I'm not sure that all potting mixes are created equally. We bought some 'organic' potting mix at a locally owned store and we are having much better results so far. The texture of the mix is also much softer and drains well, yet retains moisture. No large sticks of pieces of bark either.

I don't know what is put into miracle grow potting mix, but maybe it inhibits germination.

The temperature is the same, we start everything in the same room each year. Upper 60's and low 70's for soil temp.

I think the problem is that potting mix is generally meant for grown plants in pots, not seedlings. As plants in pots have less soil to gain nutrients from, a lot of brands infuse it with things like fertilizer and water-retaining ingredients. It's also more dense in texture to pack more into a small space (ie, a pot), which can make it difficult for little seedlings to poke through. So it may not be the brand that's the problem, but the fact that you're using potting soil.

The organic potting mix you have, on top of being softer and more fine in texture, may have less additives, or the ones they have are more agreeable with seedlings. I myself sometimes get bags of a brand of organic potting mix instead if the local store's run out of seed starting mix, though I mix it in with what I have.

Miracle Gro All-Purpose Plant Food (MG) inhibited germination of Scott grass seed. I was designing a science experiment for my nephew and had four groups of seeds from the same bag of seeds, at the same temperature, same lighting, and same water. The groups were: (1) on a paper towel without MG, (2) on a paper towel with MG, (3) on plain soil without MG, (4) on plain soil with MG.
Seeds in group 3 (without MG) germinated first and fastest. Group 1 (without MG) was 2nd. Group 2 was far slower, and Group 4 seemed almost inert.

All 4 groups were highly moist at all times.