I m sure you are writing your plist file that is there in bundle.Apple documentation says "It is not recommended to modify your bundle content after application code signed". when you build your application again it will be overridden by default plist file what you have it in your project.
You should copy your default plist file in document directory and access it for writing. this will even go off when you delete the application from device.
To copy your plist to document directory follow
NSFileManager *fileManager = [NSFileManager defaultManager];
NSError *error;
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *txtPath = [documentsDirectory stringByAppendingPathComponent:@"settings.plist"];
if ([fileManager fileExistsAtPath:txtPath] == NO) {
NSString *resourcePath = [[NSBundle mainBundle] pathForResource:@"settings" ofType:@"plist"];
[fileManager copyItemAtPath:resourcePath toPath:txtPath error:&error];
}