Here's a guess--- Sony equipment has command modes. The idea is that you may have more than one Sony DVD or VCR, and then both units may respond to the remote. So the RDR VX525 has three comand modes DVD1, DVD2, and DVD3. The remote has a similar setting.  If these command modes don't agree, then it won't work.

See page 22 of the manual to see how to match up the command modes.
If you don't have a manual, try tv.manualsonline.com/manuals/mfg/sony/rdrvx525.html.