I'd say it's your age, it can be tough to break into the rental market. Where i live it is super, super competitive and as said they will almost always choose and older couple with paying jobs (totally no offence to you guys). A lot of real estate agencies still think that 18 year olds will have parties and stuff.
Could you get your partners parents to apply for the house for you? I know that it wouldn't get you a rental record if you aren't on the lease, but perhaps you could do that and once you get a place and move in, they could try adding you to the lease? Def do a cover letter, and make sure you have everything you need (references if needed, payslips, bank statements all of that) and hand in a filled out application when you view the property. Sometimes they will just go with the first application that checks out.
Good luck! hope you get your house.