Hakemon ... well you certainly have some restrictions
The main method to use here is to "minimize the variables" ... that is ... see if you can get a good signal under optimum conditions and then add things back in and try to determine what the signal degrades. For example this might resolve the 20%-48% problem.
Given your statements about minimal purchasing ...
1) Go to http://www.antennaweb.org/
and check where the transmitting towers are relative to your location. This site will also help you determine if you are in range depending on the type of antenna you have.
2) Using only the VHF or UHF antenna (as appropriate for the station), see if you can get a good signal for each desired station with your antenna and note the antenna position for each. You can mark up the antenna.org chart if you can print it out.
3) If you can't get a good signal, then you need to see if you can go higher (in the air) .... this may mean that you need to buy cable though!
4) If you can remove the combiner (probably not) that might help for the initial tests .... but you will probably need it back in eventually
5) Make sure that you are not connected to the FM output of the combiner (there may not be an FM output on your device)
6) Make sure all of your connections are clean and tight
7) Make sure your antennas are as clear of metal as possible (e.g., consider that you may have metal studs in your walls if you are in a recently built house/apartment)
8) If you have more 300 ohm cable you might be able to build a slightly better antenna yourself ... depending on the quality of rabbit ears
Hope that this helps ... I know antenna positioning can be painful ...
PS ... the easiest way to solve the problem is with either a larger antenna (yagi in the attic) or an amplified antenna ... but I know that you didn't want to spend the money.