Not sure what to make of this one. I have a 2 story home with 2 baths positioned above each other.

In the mornings I use the upstairs toilet and shower with no problems. However, when the downstairs toilet is flushed, it fills very slowly. We've taken the tank cover off and the water coming out of the little hose is a trickle. However, later in the day, the fill is normal.

Any ideas what might cause this, even better how to fix it?

Have no idea why it would change with the time of day but the fix is to replace the seal in the fill valve or ballcock.

Which do you have

The one on the left

Turn off water st the stop valve under the toilet tank.
Reach down under the float, lift float and grasp shank. Turn cap 1/4 turn counter clockwise and lift off. Hold you hand or a cup over valve and turn on water briefly to flush out any debris in valve. Replace seal.

If that doesn't fix problem you probably have a blockage at the stop valve under the tank. Post back.

Does the pressure fluctuate in any other fixtures (even the slightest bit) in the home during these times?

Surely you are jesting Lee.

:
Nope, I really have always done it the hard way I guess. To stubborn to read instructions :)