my puppy won't walk on the lead

i have a 4 month old deaf ebt who will not walk on the lead when you 1st take her out. she puts the brakes on & won't budge.
she used 2 wear a collar but now has a harness which she seems more happy with.
shes ok when you let her off in a field & then put her back on the lead again 2 go back on the road. its mainly going from the back door 2 a field.
I have tried taking toys out with me & tried bribeing her, bribes work but she never gets any better the next day.
on the other hand its a different story when i take her out in the car. she walks fine then.
have just started taking her 2 a dog training class which is going slowly but considering she is deaf is going well.
has anybody got any tips or advice?

If she is happier with the harness stick with that, could you use it in the house eg before feeding walk her on the lead to her dish? im sure someone else with some better advise will be along shortly
ATD xxx