Training an older dog can be harder or slower due to trauma or fear but IT IS possible - old dogs CAN learn new tricks! You will want to address your dogs fear or trauma before introducing a new cue or trick.
First, be sure you are teaching your dog in a quiet (non-distracting) environment. Once your dog has mastered these skills in a quiet environment you may add in distractions. Make sure to always have a "high-value" treat your dog LOVES as a reinforcer (see "What's Your Dogs Currency" Webinar for information on what a high value treat is). Lastly, make sure to practice for at least 5 minutes everyday for 1 year before removing the "high-value" treat.
It sounds like you need to keep her on a leash in the situation when and continue practicing on leash until the recall behavior becomes more fluent. Also, make sure you are using a "high value" treat for "come" such as hot dogs, cheese, cooked chicken. Remember to take baby steps, if you go too fast, you could set her up for failure. Slow and steady wins the race!
If you want to use the clicker as a cue to come you can so you can say "come" or whistle then click and treat when she comes to you.