Torque Converter not always locking

Anyone ever had this issue? Starting Sunday, she doesn't want to lockup when at cruising speeds as she normally does. No jerks or anything, just doesn't lock in 4th even after cruising at 55 mph for a while. It isn't every drive, just sometimes.

 

check the fluid levels at temp also service the Transmission fluid if it's due. Some things clear up just with a complete fluid change. I had some troubles with the OD switch it's cleared up now.