Plants growing smaller and more. Getting despondant

I am fed up with the same thing every year. I sow various veg and a lot stay thin, wispy and straggly or either stay the same of grow smaller ! Yes smaller. I know it sounds impossible but I was given some rare spinach at a country house veg garden shop. I planted it out in thee soil and it hardly did anything. I then dug it up and planted in deep pots taking care to keep moist. They have all got smaller and smaller until the base of the stems have gone thin and dynes are looking like they are dying. I won't be even able to get seed off them for next year. I plant rocket in abundance in soil and compost with thin, whisper results. Poached egg plant started healthy and around 20 plants just one survived. Radish stay the same size for weeks with no swelling. I have some now that have been growing for weeks and they still have not swelled. I thought they were supposed to be fast growing. Bought a small pot of tarragon which has been sickly all season with brown patches. Cared for it and even now it is smaller than it was when bought in March. Basil grown from seed with the toms in the greenhouse just goes silky and brown patches on the leaves.red Sorrel I bought 3 years ago manages to push one or two leaves out per month. Smaller now then when first bought. On M&S hardly bigger than the sets i planted. Amsterdam Carrots planted in container stayed tiny, some never formed. I could go on with more examples.

I do have some success so it's not all bad but it bugs me when seemingly easy veg and salad fails every year when I do all the right things on paper. Ok, my garden is small and walled with limited sunlight but still enough for growing. It's the winter it's more limited. I get a lot of pests like flea beetle, leaf miner, damping offaphids, slugs, so its hard going.