Fill a bottle from the tap, carbonate it, and the fizz is soft and short-lived. Do the same with water straight from the fridge and it bites. Nothing about the machine changed. What changed is how much carbon dioxide the water is willing to hold, and that depends mostly on temperature.
Carbonating is just forcing CO2 gas into water under pressure. Gas dissolves in liquid more easily when the liquid is cold, which is why a warm fizzy drink goes flat faster than a cold one and why opening a warm bottle makes it foam over. At the normal air pressure of one atmosphere, water at 0 °C can dissolve about 3.3 g of CO2 per litre. At 20 °C it holds about 1.7 g, roughly half, and at 30 °C only about 1.3 g.
Raising the pressure raises the amount that dissolves in proportion (this is Henry's law: double the pressure, double the dissolved gas). So to reach the same fizz in warmer water you need a much higher pressure in the bottle. The tool below does that sum for you.
Drinks people compare by feel can be compared with one measure: volumes of CO2, meaning how many litres of gas are dissolved in one litre of liquid. One volume is about 2 g of CO2 per litre. Cola and other strongly fizzy sodas sit at roughly 3.5–4 volumes, lemon-lime sodas at about 2.5–3.5, a typical lager around 2.5, and Champagne at roughly 4.6–6. That is why the tool's three settings are 2 (light), 3.5 (classic, close to a tonic) and 4.5 (strong). Values vary by brand; these are common ranges, not a standard.
If you pour in room-temperature water and press the same number of times, you end up with less dissolved gas than the cold-water version, however hard you press. Machine makers tell you to use cold water for this reason. The practical routine is simple: keep a bottle or carafe of water in the fridge, fill it to the line, carbonate, and put it straight back.
The same physics explains the fizz in your glass. A drink that is poured over ice, or served cold in a chilled glass, keeps its bubbles noticeably longer than one served at room temperature, because the water is holding the gas more firmly.
Home machines are designed to carbonate plain water, and the makers say so clearly: do not carbonate anything else, and add syrup, juice or flavour after the water is fizzy. Sugar and fruit solids give the gas places to form bubbles all at once, so the drink foams up, can clog the machine and can spray out. Adding a splash of juice or a shrub to already-fizzy water is gentler, and you can stir it in slowly with a spoon rather than pouring from a height.
If you want a flavoured fizzy drink, make it in this order: cold water, carbonate, then flavour, and pour it down the side of a tilted glass to keep the bubbles in.
Machines differ, but the idea is the same everywhere: the first presses add most of the gas and the later ones add less, because the water gets closer to what it can hold at that temperature. Many models suggest a small number of presses for light fizz and more for strong, and you will find the right number for your taste in two or three tries. Write it down once and you will not need to measure again.
One safety point: never keep pressing past the point where the machine stops adding gas, and use only the bottles your machine is designed for, since carbonating puts the bottle under real pressure.
Three things matter at any budget. First, how the gas cylinder connects: a quick-connect cylinder swaps in seconds, and you can usually find refills from more than one source. Second, the bottle: lightweight plastic is fine for everyday use, while a glass carafe looks better on the table and does not hold flavours. Third, whether the machine suits your routine, because a model you can leave on the worktop and use in ten seconds gets used far more than one that lives in a cupboard.
A word on drinking: plain sparkling water, with fruit or a dash of bitters-style flavour, is a good way to have an alcohol-free round at home. Pair it with something you would want to drink on its own.