Before I answer, I have to say that this is just my theory.
The grass on route 15 is not optimum for shiny chaining; there's not enough of it at any one place. I've had that before with places that don't have enough grass to chain. That additionally means you frequently have little choice but to go into patches on the edge of the grass, which are liable to break your chain. I've had that happen to me once or twice in optimum chaining places like Route 7; it's just a risk with edge patches.
Also, turn on your music (if it's off) and see if you do something wrong before you reset the Pokeradar. If you do, the music will switch back to the route music in order to tell you you've somehow ended your chain. You might have accidentally walked into Route 16 or something, or selected another key item (when chaining only have the Pokeradar on your Y button). I don't know how much you know about chaining, so you might be doing something wrong and not realising it, like getting on your skates or your bike.
I think the best way to see if your Pokeradar is glitched is go into Route 5, and try to chain in that huge patch of purple grass, since that's the easiest patch to chain in. If your friends have the time, ask them to help you chain by watching you, since they'll be able to pick out what you might possibly be doing wrong.