CONFUSION!!

My first month using these was in February. It was my first month off birth control. I bought a one month supply of 10 sticks and the reader. I got the flashing smiley on day 26 and a solid smiley on day 30. Unfortunately, didn't get pregnant.

In March, I bought a 2 month supply with 20 sticks and a reader. I got a flashing smiley on day 21 and a solid smiley on day 26 using the new reader. Still didn't get pregnant.

This month, I had some sticks left over from last month so I started using those and the same reader. Started testing day 10 and got empty solid circles and got my first flashing smiley on day 12. I ran out of tests so I bought another 2 month supply. Today I opened the new box and out of curiosity, I used the same reader from last month that I've been using and the new reader. I put a stick in the old reader and put another stick in the new reader and tested using the same urine sample. The old reader gave me another flashing smiley and the new reader gave me the empty solid circle. Any ideas?! I'm now on CD 18.

Side note....I also use the cheapie line tests from Amazon for comparison and those have been consistent each month with the digital tests. They always show positive the same day as the digital readers, and I haven't had a positive yet this month on those, so I know I haven't missed the surge.

Which digital reader should I go by?!