That would be my guess, then - your signal isn't strong enough to split, or your tuners aren't able to decode the resultant signal, anyway (not all tuners are equal, some are better than others).
You could try different tuners; you could try a dual tuner (so one input to the card, which can amplify on-board); you could try an in-line amplifier (masthead would be better - getting it closer to the source amplifies more signal, getting it closer to the tuners amplifies both signal and noise). I suspect that internal tuners are better than USB ones, but I have no proof of that - they just have more room for more chips.
Really, you'd need to do some testing, either with other kit or with something that would tell you the signal quality in a more sensible way. I don't think tvheadend's signal strength indicator is really useful, as I believe it's tuner-dependent, although it would tell you something if both tuners are the same.
