Tip from the pro

In live situation, when the drums are not miked, here’s is what usually happens. We hear the bass drum and snare clear and loud as well as the hi hat if it is played with the shoulder of the stick on the side of the cymbal, or half open in a “whooshy” sound effect. The crash, most likely pounded hard at the end of a fill, also come through strongly. But the toms don’t. A fill starting on the snare and going all around the toms will disappear as soon at it hits the hi-tom.

Because the snare is a loud, very loud and distinctive instrument. That’s also because the toms, especially the hi and medium toms, have smaller shells producing less sounds. Finally, the toms are bound to blend easier with the bass or the guitar, their sound is much less noticeable than the snare.
So, what can be done?
Compensate. Play the toms louder, harder, than the snare when you do a fill. Help them carry through the mix.
Last but not least: in a rock band, the groove will most likely be played with the snare doing rim shots, which is the loudest sound you can get from that instrument. But the fill will not. So the fill will lose some of the energy instead of adding some.