Symbol-rate is a measure of the quatity of digital information per unit time that is embedded in the microwave satellite signal. It's unit is MegaSymbols per second. It is analogous to the "bit-rate" used in computer terminology. Most receivers can process from about 2 to 40 or 45, though a few receivers are limited to a small range, or even - as is the case with the so-called Sky Digiboxes - limited to a few specific values.
SCPC = Single Channel Per Carrier Type of transmission where only a part of the available transponder is used for the signal, allowing the satellite operator to sell the remaining space on the transponder to other uplinkers. SCPC is typically used for feeds rather than for direct programming, and for some channels which are not really intended for mass consumption (eg directed at cable distributors). SCPC's advantage over MCPC is that the signals uplinked to the same transponder can be transmitted up to the satellite from different locations.
MCPC = Multiple Channel Per Carrier An average satellite transponder has a bandwidth of 27 MHz, sufficient for multiple video or audio channels to be transmitted simultaneously. MCPC uses a technique called Time Division Multiplex to transmit multiple programs, which works by sending data for one channel at a certain time and then data for another channel at another time.
Many encoder manufacturers are currently experimenting with statistical multiplexing of MPEG-2 data. Using this technique, channels that need high data rate bursts in order to prevent pixelization of the picture, such as live sports events will obtain the bandwidth as they need it by reducing the data rate for other services that do not.
Most receivers can process both SCPC and MCPC signals. Again, though, there are a few "crippled" receivers that can only process MCPC (and again, the Sky Digibox is one such example).
Symbol rate depends on the bandwidth used. In DVB-S transmissions (that use QPSK) one symbol means 2 bits. So, (bit rate)=(symbol rate)*2. This is the total bit rate. The useful bit rate (used for the services and the SI tables) is less than the total bit rate. The rest is the error correction data. (total bit rate)=(useful bit rate)+(error correction bit rate)
SCPC carriers are used for fixed satellite services. One broadcaster can send material to another using an SCPC transmission. The commercial transmissions are MCPC.
In the past, some receivers were incapable of receiving low bit rate signals. Some old receivers could support 16-45MSps (for example).
If the carrier has a low symbol rate (not 27500 or 30000 but 4123, 3666 etc) then the total bandwidth of the carrier is also low. If the Local Oscillator of the LNB is not accurate then it may be difficult to receive. You may have to "play" with the frequency (increase or decrease it). I have faced a problem like this. Although i had a professional LNB, i had to set the frequency to a higher value (+5MHz).