TL866A programmer in-circuit

[keenox]

Hello,

I recently bought a TL866A programmer and was playing with it. I noticed it cannot read EEPROMs in-circuit. With the chips desoldered, it works no problems.
I also tried powering the circuits, but it still doesn't work. It doesn't say OC protection, it simply reads only FFs.
With a cheap CH314 programmer it works in circuit no problems.

Am I doing something wrong or the TL866A is incapable of reading/writing in-circuit? If so, why would it be a problem?

Thanks!