Something has to be wrong for that amount of leakdown, especially with a starting pressure of 75psi. Shraeder valves are very reliable but it is possible for the core to be loose. Any bike shop or service station should have a tool to tighten the core. A bit of very soapy water in the stem with it pointed upward will easily show if it's leaking. The odds of that on two random tubes is very low. I would recommend inflating the tubes quite a bit and putting them into water to check for leaks.

It's also not clear what you mean by "deflate 10 psi and 15 psi after every ride." I would not want to see that much in 24 hours but it's conceivable - from beginning to the end of a ride is a whole different matter. I am assuming here that you are using standard tubes, not ultralight and certainly not latex.

Finally, how are you measuring pressure? If you attach your tire to a pump with a reservoir part of the air in the tire will go into the pump reservoir, giving an artificially low pressure.