Flat hair and outward-growing hair need different conversations
If your hair becomes flat as it gets longer, asking for more length may not solve the problem you came in with. A shorter top with a taper has been suggested for that situation, with the remaining hair directed forward, upward, or to a side. Treat those directions as options to check on your own hair, rather than a guarantee of all-day volume.
Very straight hair that sticks outward at a short length presents a different problem. Keeping it short and letting it grow are competing choices. During a grow-out, ask whether the top can be left alone while the sides are maintained. That approach has been used to get past an awkward stage, but it does not promise that every part of your hair will settle at the same time.
The useful distinction is what you want to change. If the top collapses, show the barber that loss of shape; if it sticks up, show where it happens. These are opposite frustrations, even when both wearers describe their haircut as short. Do not let a recommendation to grow the top become an automatic answer to a top that already falls flat.
Be specific about when the flatness bothers you, too. Hair that looks acceptable after styling but loses its shape when you run a hand through it is a different brief from hair that never lies down. If the style also becomes dense and flat between washes, describe that change. You may want a top you can redirect after it is disturbed, rather than a taller finish that you have to preserve untouched. That is a practical reason to discuss a shorter, versatile arrangement instead of simply adding length.