Doing a pH test at different levels is a good idea but while acidity underneath may explain poor long term growth it will not explain bad germination. Non digging over a number of years can sometimes result in an acid surface layer due to leaching and organic matter rotting on the surface.
You may well have soil pests that are not obvious. I would put my money on pests rather than any soil problem. I may be wrong.
Apart from the usual suspects such as wireworms, millepedes, leatherjackets and slugs, there are tiny invertibrates such as symphalids and springtails which sometimes feed on the root hairs of germinating seedlings. They usually live peacefully in the soil living on rotting vegetable matter.
I know a little about symphalids and less about springtails. Google images will show you what they look like
Symphalids are white and shiny, about 3mm long and can be mistaken for small millepedes. Most of the year they are deep in the soil but in the Spring, when the soil is warm and moist, they move into the top layers where they feed on young roots. In small numbers they do little harm. Where and when they appear is unpredictable but sometimes they are in hugh numbers and can decimate crops.
Take a shovel of soil and drop it into a bucket of water. Pests will usually float to the top.
Pests can be very fickle and straight lines can easily occur due to slight differences in moisture, pH or organic matter.