Looking at your flowchart in the original post, you have an Unpacked to Packed block after the demodulator, with bit's per symbol of 1. This doesn't seem right to me. I have never tried this with a random source like you have it setup, however there should be an Unpacked to Packed block prior to the modulator and a Packed to Unpacked block after the demodulator. These should also have bits per chunck values that correspond with the bits per symbol of the modulator and demodulator. You need to feed in the data in a chunk with the correct amount of bits that correspond to the bits per symbol of the modulation scheme being used. In the example, it looks like you are using QPSK, and therefore the bits per chunk should be 2 (which is log2(number of constellation points)). The modulator and demodulator work with chunks of data where each chuck corresponds to a symbol.
For the Samples per Symbol, if I were transmitting over the air, I would raise this value to a little bit more than 2, just to ensure the receiver can lock onto the changes with given noise before the symbol changes again. In this case of looping the modulator strait into the demodulator, this should work fine.
One more thing I would look at would be the Error Rate block source. I have never used this block, but in my thinking about it in this flowchart, I would source it from the throttle instead of the random source. This may help with keeping the data a little more somewhat aligned.