Firstly, you can check
here to get a list of local transmitters, their power, distance from your property, and a suggestion as to what type of antenna to use.
If you have more than one local Tx, it might be that you're simply pointing at the wrong one to receive Freeview channels. For example, you might have a local analogue repeater which would provide you with perfectly acceptable analogue channels, but no digital reception at all.
Assuming you're pointing the right way, another problem could be incorrect polarization. AFAIK most transmitters have their antennas in the horizontal plane, but it may be worth checking up just to be sure that you don't need to have your aerial in the vertical (i.e. spin it round 90 degrees so the elements point up and down instead of left and right).
Also, if you already have a masthead amp and the cable run is short, you might do more harm than good by sticking another amplified splitter on the end of the cable. Too much signal can cause just as many problems as lack of it, so if you need to get a signal to multiple TVs then you may just be better off with a good quality passive splitter at the end of the downlead.
If the cable run is long then fair enough, put an amplified splitter on the end by all means, but I can't imagine any cable run as 'lossy' as that in a domestic situation unless it was faulty or seriously cheap cable.