Are you sure you have the right dipstick in the trans
I know 4l60e have shallow and a deep pans, mine has a deep pan which has "deep pan only" (or something like that) written on the dip stick, shallow pans have different dipsticks
I have no idea if your trans has different pan depths options
Is your pan OEM or aftermarket?
When was your trans last serviced?
Did they use the correct filter, as with the different pans, they use different filters, a shallow pan filter will not sit down deep enough in a deep pan which could, may, result in it sucking air???, maybe....