Hi OP.
Years of fertility treatment here so I've got a bit of experience.
The thing to understand about ovulation tests is that they detect a surge in hormones which can for some women be a window as small as a couple of hours. Some people find it is difficult to catch that tiny window.
I don't think the brand particularly matters and I would recommend against going out and spending loads of money on different brands - they mostly do the same thing and there's a lot of marketing in fertility.
If you are someone who has a small window, ovulation sticks might just not be the right thing for you, and you really don't want to get into a situation where you become obsessive about testing - that way lies madness!
It's also important to note that, even if you do detect a surge, you don't have to have sex literally in those couple of hours.
There is a fairly large window when you are fertile really - as long as you are having sex regularly and in days surrounding your ovulation, you will have a good chance of getting pregnant.
There are other indicators that you are ovulating or close to it, such as increase in what they call 'egg white mucus'/ discharge, increased libido, etc - look out for those as well.
You don't have to catch your exact surge. Sperm can live for around 5 days so even if you had sex a few days before your surge, or up to a day or two after, you can get pregnant.
How old are you both? If you're under 35 and otherwise healthy, then it's a bit unusual that you've had unprotected sex for 2 years (even if not that frequent) and haven't got pregnant at some point.
I would really advise you just both go and get a fertility MOT as that will give you a starting point to work from. It's better to just go and get the facts from a professional than faff around with home testing and trying to figure things out yourself.