First dog won't go outside

We have two dogs, the first one is four years old and a Jack Russell mix. We got our second one around 6 months ago and he is 6 month old miniature daschund. Tina our first dog went out in the garden whenever she needed to do her business between walks, but lately she will not go out in the garden and holds it in until it just seems to fall out when she walks around the house. Does anyone have any theories as to why this might be. We thought it might be something to do with Charlie's scent because he goes in the garden also, more so than Tina. When she goes out she acts as normal doing her business as normal. Please can someone help me as Tina is obviously getting very upset as she is normally very clean in the house.:

are you sure Tina is in good health? no UTI? a vet check may be a good first step.

If all is clear, I would take Tina out to the garden ON LEASH at regular intervals, and praise/treat when she goes. don't come back in until she DOES go. and praise her to high heaven when she does. basically, treat her like you would a puppy.

sometimes does need reminders on training, but honestly, I'd head to the vet first, to rule out any medical issues.

Yes I agree with Jessi.

I would get a check up from the vet first to rule out any health problems.

Then with an all clear from the vet, by bringing her out on the leash she will get used to that idea again.