This is a very rare situation that we have come across once before with a StarFish set-up.
The customer wanted to boil water in 5 flasks, but reported that the water could not reach 100°C and was not boiling, despite the hotplate temperature being high.
Unusually, they were not using condensers, or stirring.
After investigation, we discovered that the cause was open side arms of the flasks they were using. With the open side arms, there was a lot of evaporation, which cooled the water, and meant there was water loss without boiling. When the side arms were stoppered, the water was able to boil.
Another potential cause if you are having issues reaching temperature is poor contact between the flask and well, e.g. if you are not using the correct insert for your flask size, or if your flask is misshapen. It’s also possible you have not set the hotplate temperature high enough – it needs to be above the boiling point, as there will be some heat losses.