Ruby
You are trying to instill three separate lessons simultaneously. It may be better at her age to break them down into the individual components.
To work the drop here are some suggestions;
First get her up on a training table. If you don't have a training table, it can be made very cheaply and inexpensively out of 1/2" exterior plywood and three 2x4's. Make a table 8 feet long and 2 feet wide. Frame it with the 2x4's and put 4 legs on it. It should be wiggly, but not rickety. You want the dogs attention on you. The purpose of the table is twofold. It gets the dog up off the ground and out of it's comfort zone, and it makes the dog easier to get to by bring it closer to you. (The tailgate of a pickup truck work just fine too.)
Keep your dog on a short leash and give her the dummy, each time by hand and then make her give it back, or drop it. You are going to do this a lot, over an extended period of time. It's not a one time deal. In the beginning use her favorite toy or object. You may have to take the object from her by pinching her gums into her back teeth while commanding "drop", and taking the object. You aren't trying to teach the "Force Retrieve", she's too young for that, you're just trying to instill some manners. Be gentle and patient, and always end on a positive note with some playtime. Lots of praise and reward.
Since she does bring objects back to you, she has the retrieving instinct already developing. She isn't going to be 100% at 6 months, but she'll get better and more consistent with time.
Put her on a 50 foot lead and toss her favorite dummy out there, within the range of the lead. Give her the command to "Come", when she doesn't come, gently reel her in while reinforcing the come command. Bring her all the way to hand and then command the "drop". If she starts to return on her own stop reeling her in and let her come on her own. In the beginning you'll be reeling her all the way to you, and over time she'll start to come on her own. Of course she'll be sneaky once she feels the slack in the lead and she'll attempt to go a different direction thus faking you out, but you'll be ready for it.
The tennis ball is a small object, and without being there I don't know if she just interested in the ball, or is not "Marking it", meaning she sees where the ball went, or fell. Some dogs aren't as good at marking as others. If she is losing sight of the ball when thrown in the air this could part of the problem. She doesn't know to look for it, because she doesn't know where it went. She'll get better with age and experience.
If you throw the ball so that it skips along the ground in front of her, a "rabbit toss", and she follows it easily she may just need more experience" Marking". To work at this, hold her collar and toss the ball in the air so that it lands a short in front of her. She'll soon learn to stop following the motion of your arm in anticipation, and start watching the ball. Make them really easy tosses. Release her for the retrieve and the drop.
The dummy must have some sort of a scent to it that she likes which encourages her to search for it, whereas a tennis ball has no scent. You could buy a bottle of Quail, or Pheasant scent and put a few drops on the tennis balls to encourage her.
She's still young though, and even with a hunting dog it takes a fair bit of time to develop them into a no slip retriever. Give her time and experience, and she'll be fine.
