What size gel humidifier do you have?

You can always try a bigger one or supplement with a second one. Winter is hard on humidors. Heaters really suck the humidity out of the air inside your house. Its a very common complaint during winter months that a humidor is hovering below the ideal RH.

If you are sure your seal is good and you are refilling your element often enough then you need more surface area. IE another element.