A couple of possible reasons:
1) wrong logs: the logs are configured as scenery, and not loads;
2) incorrect configuration of the logging cars: even if the logs are configured as products, if the product queues have not been set up to load the specific logs you have, they won't load; and
3) wrong loading point: the cars may have different points for loading the logs than they have for loading lumber, so if you're using the wrong loading point, the logs won't load.
These are not the only reasons, but better answers would require knowing the KUID numbers of the cars and the logs.
Note, however, that prototype railroads rarely use the same car to handle logs inbound and lumber outbound. A more likely prototype scenario is for the logs to arrive on one type of car, which is unloaded, and sent back empty, and to load outbound lumber of other cars, which are received empty, and released loaded.
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