What kind of tube is it? If it's latex you will lose enough pressure over 24 hours to be VERY noticeable. The standard butyl tube will also diffuse air over time which will also result in a low/flat tyre. This is normal for all tubes. If you use Co2 to fill your tubes it will diffuse even faster. That being said there is a normal rate of air loss and a abnormal rate. My tyres usually lose about 10-20psi/week with butyl. Same per day with latex.