Some newer phone chargers are smart Chargers and require the device to request a power level in order to give it to the phone if it fails to receive the request for the higher power level it will default to a lower charging state though your charger is advertised at 25 watts it may default to a lower level if the device is not sending the request signal or if something is causing it to fail to receive the signal such as a cable that is missing one or more pins that the signal would be sent on or if the connection is loose it is also possible though less likely that the charger itself could be defective if the 15 watt charger is charging faster than the 25-watt one then it is likely that the 15 watt charger is receiving the signal to send full power and the 25-watt charger is not receiving the signal try switching the cables if possible
If the phone's internal charging circuit is not designed to run at 25 watts then attempting to force it to do so could fry it, however it has been stated by Samsung's president that an update will be coming in the near future to enable 25 watt faster charging for the s10's. So just be patient and periodically check your phone for updates under the settings. I personally would be hesitant though because from personal experience faster charging a battery can sometimes result in a shorter life span, overheating and bloating or popping. In computer repair we say that Apple's(intel) "Thunderbolt" circuitry it's called such because it tends to fail and looks like it's been struck by lightning when it does due to excessive wattage
Samsung China Division OFFICIALLY said that Samsung would be updating the entire S10 series to allow for 25W charging, but I haven't heard anything since. I'm assuming they software locked the S10 series (except the 5G version), so they could easily do this.
Hopefully they unlock it in the next few months, because the charging speed is my main complaint against the S10 (I have the S10+), especially when compared to their competitors