We just went over this whole issue in my puppy socialization class last night. I dont know how old you cav is, but here are what we were told to work on to set the stage to learn come (for older dogs this will be too babyish).
The start was about everything Cathy Moon just told you - never use the come word for things your dog doesnt like (ie. if you dog likes the car you say come - associates come with nice things but if your dog doesnt like the crate in the car - go get the dog dont call him to come as he will associate it with things he doesnt like). She stressed the point to come was that whether in a few days, few wks or few yrs all dog owners will hit a moment Come will save their dogs life. To not use it for anything that will not produce positive reinforcement or failure of the command.
Start off with any games that involve the puppy working his way back to you - fetch, hide-seek using high motivation reward (for us its chicken!). When you got that down, you up the anti by randomly calling your dog throughout the day (still using high reward) like when they are in the garden or engrossed in a toy - just call whoever come, and all they know is that means big time treat) Finally you take your dog to an enclosed unknown area (suggestions tennis court gated, neighbours backyard, kindergarden gated play area etc..) Let your dog wander about, using a large toy that can shake and make a noise you wave it/squeek in to get attention as they are over at the other side and call excitedly - reward with that chicken. She said if need be you can use the long cord route if difficulty is happening but to avoid pulling - rather a short burst of tension and show treat to get them to move a bit reward - little more. If it turns into a pull match or total reel in, stop.
For me I cant get kodee to play fetch so I was stumped - she is typical puppy and just goes and gets the toy, comes back but jumps about me. She suggested having 2 balls - kodee comes prancing around me, hold out other ball with treat tucked in hand and she drops it, reward then exchange balls. For advanced hide and seek you put you dog in a stay and hide the treat and say find it and when they finish eating say come, reward again, put in sit/stay and go again.
Dont know if any of that helps at your stage - but I sure found it alot more informative then the methods taught for come 15 yrs ago last time I did dog training! I definately over used come with our dog for all sorts of things so I can see why it was not 100% recall ever in her. But then cookie was always 100% recall :lol: and that word saved her butt a few times she got out the door! I swear the neighbours thought my dog's name was Cookie!