I had the exact same thing happen. Tried flushing the fluid and solenoid. Nothing worked, finally took it to La Mesa RV. I will have to dig out the invoice for the verbiage, But they replace the solenoid for that slide. They said it leaked over a period of time, causing the slide ram to leak as well. They replaced that ram and I have not had a problem since.
2014 Excel Limited 34IKE
2015 Ford F350