DPL255 is not a standard DPL code, i.e., it's not part of Motorola's set of 83 standard codes. However, it is part of the set of 104 standard codes of which Moto's 83 codes is a subset.
I recommend picking a code from Moto's set of 83 standard codes to help guarantee using a code that works plus have compatibility across different radio manufacturers. I'm using 411 in my local Nashua repeater (note VERY local coverage, I play around with it at home).
I had to eventually step up to the plate and use DPL instead of PL in my repeater because it's getting hard to find a PL tone that's "clean" in the area. Given that my repeater is somewhat portable and I take it to the major flea markets, I'm less likely to bump into another GMRS repeater in range of the flea market that uses the same PL I'm using. DPL solved all these issues. I had my repeater on the air from my car at Boxboro 2008 over the past weekend and was using it with my G/F. Worked great!
Try 244 instead of 255. DPL244 is a standard Moto code and part of Moto's set of 83 codes.
Also a word about nomenclature....the terms you'll hear are DPL and DCS. Motorola trademarked DPL, so the other radio manufacturers have to call it something different. Digital Coded Squelch (DCS) is the most common name for it in non-Moto radios even though they are the exact same thing. Basically, if it's in a Moto radio, it's DPL. If it's not Motorola, it's not DPL. Got it?
Motorola DPL information:
http://onfreq.com/syntorx/syntorx/pldplx.html
Good technical discussion of DPL operation:
http://onfreq.com/syntorx/dcs.html
This is the best and most complete tech discussion of DPL I've been able to find on the web.