In the doorway of your dryer, there’s a filter. You should clean this lint filter properly. Clean the filters each time before you use the dryer and make sure these filters don’t completely ‘close’. If this is the case, you should soak the filters in vinegar. Mare sure all openings of the filter are free of dust.
Drains of a dryer have hose that leads to the sewer. A lot of dust can accumulate in this hose. Please check regularly if this hose is free of dust. Also check the vent in the dryer. Again, a lot of dust can accumulate which may slow down the drying process. Also clean the passages behind the filters and free them of dust.
The laundry is working at its best if the hot air is able to circulate during the drying process. By putting too much laundry in your dryer, there is little air movement and it may take longer for the laundry to be completely dry. Make sure that the drum of the dryer it only 60 % full, so keep some space of above the laundry. This gives better drying results and also saves energy.
Probably the air/water drainage is blocked at some point and the hot air/humid air can’t be discharged. The humid air will still with the laundry, causing to not dry or dry badly.In case of malfunctioning, please check your user manual.