It's only been a month since your surgery, Veronica. Your ROM is fine for this stage in your recovery. There's no need to worry about it, because ROM can continue to improve for a year, or even longer, after a knee replacement.
It often takes longer to achieve straightening (extension) than it does to achieve bending (flexion). Just let your knee take its time. It's still in the very early stage of healing right now. I'm sure that you're going to achieve all the ROM that you want - but it's not going to happen quickly.