You state a 'splitter' is in use, If you are indeed using a cable splitter it will never work, you need to use a multiswitch, satellite works alot different.
It sounds like you have a dual lnb dish (2 cables from dish into a multiswitch then (usually 4(or more)) outputs from switch to each tuner), and since you say you have 3 outputs in use (2 for dual tuner, 1 for single), then you probably have a 3x4 multiswitch already by the dish, if this is the case you would have one open output from the four avail (3 in use) on the multiswitch, you would have to run rg6 from it to the walljack (or direct to receiver), no splitters can be in the mix, nor anything used for cabletv like attenuators, boosters, or filters, the cable should also be rg6 (not rg5). It could also be a fault in the runs to the plates. The installer should know this.
Hope that helps.