Coming when called is sooo super important and probably one of the hardest things to teach, to a terrier anyways. That's what I've found.
I am still working with Jackson continually.
I try to use a REALLY rewarding treat like hot dog, cheese, chicken, steak, etc when practicing coming when called. I bought a 50foot training lead (Petsmart has these for around $10-$12) and I let him go all the way out 50 feet and then call him to me... so it's almost like he's off leash, but IF he happened to run off, he's still safe at the end of that 50 feet. I also found he responds better to "here" than "come". I don't know why. So you can try different words and phrases.
I'm lucky that my dad has a big fenced in yard and we also have a nice dog park. Both are nice places to practice recall. I always keep a few treats in my pocket at the dog park and will randomly say "Jackson, here!" and he responds REALLY well most of the time now at the dog park (with lots of distractions). It's been a nice place to incorporate training. As well as agility class.
Basically: Practice, practice, practice! Don't use the word "come" or "here" if you know they are NOT going to come. Set them up for success. If you overuse any word, they become 'immune' to it and don't really understand the meaning behind the word. If you keep saying "Come! Come! Come!" and they're not coming... that word is worthless. ONLY say 'come!' when you know they are going to, or after they do.
The key is making YOU be exciting. Make it so your dog wants to come to you, and has fun while doing it. There's also a few things you can do in emergency situations like get on the ground, so that your dog looks at you and is curious as to what you are doing and comes over to explore. I saw this on It's Me or the Dog.