if you're getting 4 solid symbol lines but no decode this usually means the decoder was not able to locate a frame sync (FS) sequence in the demodulated dibit stream.  The most common causes of this are due to anomalies in reception, which can be sorted if you check the dibit stream for a few of the common ways the FS can get mangled (all sequences shown are hexadecimal)

0x5575F5FF77FF - normal FS
0xffdf5f55dd55 - signal is inverted (common when using a disctap, due to the superhet principle)
0x001050551155 - frequency incorrectly tuned 1,200 Hz too low
0xFFEFAFAAEEAA - frequency incorrectly tuned 1,200 Hz too high
0xAA8A0A008800 - frequency incorrectly tuned (error is +/- 2,400 Hz)

All of these errors are more or less quasi-hardware related. The inverted FS should be caught (depending on which software demod you use) and is usually only an issue with hardware receivers and the latter three errors are a consequence of tuning error when using the DQPSK demod, suggesting a need to tighten up your frequency correction (a.k.a. PPM, offset, calibration, etc).

Max