The reasons your pH is not going up is probably:

1. An indoor pool is not seeing sunlight, so the chlorine is not being lost through UV light from the sun.

2. You probably have a light bather load.

3. The pH will eventually start to rise, so you will be adding Muriatic acid to compensate, but not as much as when having an outdoor pool. This is part of the SWCG process.

4. Be glad you have what you have.

Hope this helps.

Pat