Radiocarbon dating isn’t as simple as most people think because the ratios of the various carbon isotopes varies significantly by region and time period. A volcanic eruption somewhere else on the continent, for example, can dump ancient C14-poor carbon from underground which changes the ratio of C-14 to other isotopes, making samples appear much older than they really are.

They need to come up with a calibration standard for the locality by measuring a bunch of other samples that have been dated by other means to establish a control. Then they will be able to actually date the cave painting. Calibration standards are hard work and it might take a while.