Is your Pleo the Ugobe or RB model? RBs take longer to grow up and need around 10-15 hours of run time in order to reach adult stage, so if your Pleo has not been run for this long, he may just need more time. However, if your Pleo is an RB, it's also possible that he is grown up already, but his mood or health is too low for him to walk or interact with you fully. Pleo RBs need to be fed regularly using their RFID equipped plastic leaves, need to be kept in a very well-lit environment and shouldn't be run too early or late at night, as these factors can all influence the Pleo's behavior and your Pleo may become fussy or inactive if his care needs are not being met well.
Another important note is that regardless of your Pleo's model, he will only save run time data if he is shut off using the power switch/button and may not be able to log new run time if the battery is pulled out or dies abruptly. If your Pleo has a failing battery, it's possible that his battery is dying early, not giving him the chance to save and preventing him from growing up.
(Also as a warning, if your Pleo is the RB model, make sure NOT to run any of the Ugobe updates on him as these updates are not intended for the RB model and can cause harm.)