There are some grotty composts around but you could only be sure that was the cause of the failure by growing similar seeds in a different compost. The council composted waste ones are OK for potting on larger plants but they have too much coarse material for seed sowing. No idea if branded composts use use council waste but agree they can have too much big stuff in them. Next year try one labelled Seed Sowing Compost and see if its any better.
Have to confess I just use a good ordinary one and find Humax is satisfactory, and a good compromise if you haven't got a lot of seeds to grow. I've never found coarse bits in it.

I had a problem once with council recycled waste & I'm sure it's because of all the stuff in it,ie- rotted evergreen
clippings & Rhododendron. Now I just buy branded compost. As Stera says, use a seed compost for your showings next time then pot on.

I tend to agree with Steragram, and best to buy a quality, and I'm with peat-based composts for seeds and cuttings. I tend to boost ordinary composts which I use for planters and baskets for summer displays with slow-release granules. I would very much doubt that branded composts would use garden waste but more likely to use shredded wood or composted straw to bulk the mix out.

I think a seed starting mix is the way to go. When the plants mature to a decent size and you pot them up, then use the compost.

I started using Vermicompost I make myself at home. Actually the earthworms make if for me. I just feed them kitchen scraps. That works pretty good.

Details matter, too. I have dealt with many customers who complain of how poorly their potted plants did. When they finally admit, or they proudly present the bag, their "potting soil"--as we call it here in the States--turns out to be planting mix or soil conditioner. Those are entirely different products intended to be mixed into poor soil to increase the organic content, and make lousy potting compost. So, double check exactly what the bag said.
Also note that plants growing in potting compost will need much more consistent and frequent feeding than plants in the ground: "occasional" may not be enough.

I wouldn't have thought the proportion of rhododendron cuttings in all the green waste would have enough inhibitory effect. I do get the council freebies but I use it as a soil conditioner rather than a potting compost.

And do remember that the nutrients in any compost do not last much more than a few weeks after using. That is one of the reasons for potting things on into fresh compost.
Also the watering and feeding regime for non-peat is different. They tend to dry out quicker than peat based ones and do not hold on to nutrients as well either. The trials results showed that feeding plants in non-peat composts was much more critical.

Cabbage seedlings surely shouldn't need feeding at 1" high though! I boost compost with slow release granules later but not at first - there is enough fertilizer in bought potting compost to last six weeks and more isn't always better.

Thanks everybody for your replies and advice. I do mix the bagged compost with my homegrown compost, leafmould, etc., but towards the end of the container filling process I only had the bagged version left.

This was the mixture which produced the worst results, which makes me think it was something in the bags.

They were 3 bags for £10 and I have now learned my lesson regarding 'bargain' compost.

Not so much what was in the bags as what wasn't. Bargain composts are made in such a way that they have very little NPK value--little or no "green" component while composting, and/or no nutrient supplements when bagged. That is the type of potting compost where cabbage seedlings need feeding at 1" tall!

Lol Tug - I said shouldn't not don't!!

I agree, Stera! Back in the 60's, when I first started gardening, practically all "potting soils" were nutritional nulls, especially the few seed starting mixes on the market. One of the first steps in the process of starting seeds was to water the tray with a light solution of liquid plant food, and do that every week until they were ready to be pricked out. Times have changed some, but the bargain brands will carry on tradition--or the worst parts of it, anyway!

It really ought to tell you on the bag about nutritional content but i guess if your product didn't have any you wouldn't publicise that fact...