[RTTY] fluttered signals

[Jacques RAMBAUD]

Hello , 
I am new to this list, looked at the archives but have not got the answer I was looking for:
Last week chazing T33 on RTTY we had the following problem:
T33C was at least 57, noise 52 but not readable probably too much multipath or fluttered signals,or ??? on MMTTY i have tryed all the possible settings , fluttered signal , multipath , tryed several other soundcard program , all about the same, nothing worked untill propagation changes and in a few (10) minutes T33 was easy to copy. it happen wery often but this time it was much more spectacular .
Here are my questions:
-what causes the signal to be so difficult to copy ? multipath ?....?
-Why is MMTTY unable to deal with such signal?
-Is there any other way , or RTTY decoder that are better in this respect , how old TU with discrete filters (88mh) works ?
-Are the KAM or PK232 better ?
any litterature on the subject?
 Thank you 
73
Jacques F6BKI  296 countries worked on RTTY