I had exactly the same problem with kutya when she was about 12 months. Until then her recall was fantastic, responding to both the whistle and calling her. Then she became inconsistent, she would come back some times and not others. She was worse in open spaces like the park, so I started avoiding going there and took her on country walks.
She was ok there initially and then again became inconsistent. She would always come back during the walk, I always praised her and gave her a treat and carried on but if something got her attention -a dead rabbit, other dogs or even just a far off gunshot - she was like a headless chicken and just would not come back to me. She would come near me but not near enough to catch! However, at the end of the walk she would just not come anywhere near me!
I then started putting her on the lead before the end of the walk to trick her! So didn't have any problems at the end!
The other day I went out and long story short she was a nightmare! 2 hours of trying to get hold of her which resulted in me being shouted at by a farmer for having an out of control dog ... Basically to the tune of "get it on a lead and don't let if off until its trained , you're ruining that dog it's a good dog"
I did go home and feel sorry for myself for about half an hour and then realised it was harsh but the best advice I could have been given! I needed that shouting at!
Kutya is now 19months old and he is right that she should be trained! I have not been addressing the problem just avoiding it! I went straight out and bought a retractable lead. And started going back to the basics at home.
Basically we feed her last, ignore her when we first get in, and practise basic commands all the time. Within a few days there is a noticeable difference in her body language and behaviour. She has had several walks on the lead and I reinforce all the time the recall command.
Now I feel an idiot! I knew that vizslas were intelligent and needed a lot of handling and had not noticed that I was letting her dominate ! Ad choose when to come back!
Honestly my advice would be to get a firm hold now! It will get worse otherwise!
I am going to seek out the farmer and thank him!