I'm far from being an expert but In my my view, Colc08 is spot on with his advice about hunting for tennis balls.
Getting a spaniel to work at close quarters, especially when there's scent around, is very often the most difficult thing to achieve. I was always told to teach a dog to work his ground at no more than 5 paces away because invariably they will pull ahead when there's game around.
Some basic rules I was always taught:
1. Teach the dog to turn on the whistle (2 short 'peep peeps' is usual)
2. Once the dog understands the 'turn' command, ensure it obeys IMMEDIATELY. Do not keep whistling in the hope that it will eventually listen - you will simply be training it to ignore you (if it does ignore you, get out straight away and administer appropriate correction);
3. Don't work your dog in straight lines. Keep changing direction regularly. It should ALWAYS be watching you out the corner of its eye
4. Keep the exercise short. 5 mins (or less) can be ample for a young dog before it loses interest/concentration
5. Don't take your dog for long walks off the lead. It will only learn that pulling ahead and taking a line is actually great fun!
If you haven't already, I would strongly advise having some lessons. Preferably with a trainer that works spaniels. You can read all the books, watch all the videos but there really is no replacement for having someone show you the ropes first hand. They will probably also point out lots of things you're doing wrong that you weren't even aware of!
14 months is still young and a good trainer will help take you and your dog to the next level.
Best of luck!