There could be a lot of reasons, so it’s difficult to tell, but I’ll list some of them below. Note that a basic characteristic of squash is that they produce male and female flowers. They look mostly the same, but the males are on stems and make pollen, while the female flowers are on the ends of the immature fruit and need pollen. More detail below.

1. It may just be too early in the plant’s life. If the plant is still fairly small, it tends to “focus” more on growth that producing fruit. It may produce mostly male flowers for a while until it moves into its “fruiting” mode.
2. The plant may not be getting enough heat, light, nutrients or water. You can generally tell this from the size of the plant, relative to how long ago you planted it. Zucchini plants love heat and sunlight, and need a good amount of water. They’re also “heavy feeders”, meaning that they may need supplemental fertilizer. I use a balanced organic fertilizer, usually something like 4–4–4. If you’re providing plenty of these, and it’s been more than a month since you planted it, then in theory the plant should be pretty big, with lots of big foliage.
3. If you see little “fruit” develop, but they don’t mature, then they’re not getting pollinated. Because of the giant leaves of the plant, insects don’t always see the blossoms to pollinate the fruit. You can take a cotton swab, find one or more male blooms (see above), and collect pollen on the swab. Then find the female blooms on the ends of the fruit and deposit the pollen on the part that sticks out from the center of the bloom. Only do this for blossoms that are open. Note that squash plants tend to open their blooms in the morning and close them by mid-day.
4. You could have other problems, but they’re generally more obvious. Powdery mildew is a common problem. Don’t mistake the natural white-ish streaks on many zucchini leaves for mildew. Powdery mildew is blatantly powdery looking, and the leaves turn yellow. This is difficult battle to fight, and a long subject. Same with insects like squash bugs. Usually, pests and diseases are fairly obvious because the plant starts to look terrible.

You can reduce the chances of getting mildew and some pests by heavily mulching around the plants, and by trimming away the bottom-most leaves, or any that touch the ground. Don’t get too carried away pruning - be sure to leave enough leaves so the plant can get energy. You can search for good videos on this on youtube using searches like “prune zucchini maximize yield”. This is a good example:

È normale che una pianta di zucchine abbia solo un frutto?

Dipende da numerose variabili, tra cui il clima, le condizioni di crescita e la disponibilità di impollinatori.

Le piante di zucchine producono sia fiori maschili che femminili, ed entrambi devono essere presenti sulla pianta per produrre. Se questo non avviene in modo efficace, le vostre piante produrranno meno frutti. Il polline del fiore maschile deve entrare in contatto con quello femminile o i fiori cadranno prima che il frutto cominci a crescere correttamente.

Quest'anno le condizioni di crescita nel mio piccolo pezzo di Texas settentrionale sono state meno che ideali (un sacco di pioggia all'inizio e poi un rapido aumento del calore). La mia singola pianta di zucchine ha prodotto diversi frutti che iniziano a raggrinzire prima che siano più di tre pollici di lunghezza - così li ho appena raccolti così.

Un colpevole sembra essere la mancanza di impollinatori - non a causa di me (non ho messo nulla ma compost e bokashi juice on my garden since we moved in twenty years ago—due to increased pesticide spraying in the neighborhood). Our half acre is the only one within three blocks with any fireflies, and those have vastly diminished in recent years. Butterflies are all but absent, despite abundant attractors. So be sure you’re not getting rid of beneficial insects.

Now my zucchini has expired, after being guillotined by some varmint overnight (not sure what), but it’s left me one final fruit, which I’ll enjoy in its memory.

So, harvest what you can, and try hand pollinating (amusingly sexy). That might help. I’ll plant another crop and do the same.