To answer you fully, I lack too much information, I should see the garden and know how you have cultivated it in past years. However, I try to make some plausible hypotheses, it's up to you to verify them.
Why a part of the garden is not productive
If a garden produces only part of it, it is clear that there is some adverse condition in the less productive area. I make some assumptions.
- Lack of sun exposure. If the side of the vegetable garden that does not produce is shaded for most of the day, this may be the cause of its low yield. In fact, without light the plants struggle to grow and the fruits to ripen. In this case, it is advisable to choose to plant only crops that do not suffer from partial shade exposure.
- Land too exploited. A land produces little if it is exploited too much. If in one of the gardens you have grown demanding vegetables (for example pumpkins, tomatoes, peppers, potatoes, courgettes, ...) for consecutive years, it is normal that they then give disappointing results. We need a good crop rotation, which includes the cultivation of legumes and possibly rest periods. Also every year it is important to fertilize.
- Problems in the ground. You may have the soil infested with parasites, such as root-knot nematodes.
Therefore, I advise you to check these three things, then if you still have doubts to try to analyze the soil of the productive and non-productive part and make a comparison, some analyzes, such as the pH measurement, can be done in a very simple way.
I hope I was helpful, a greeting and good crops!