After installing the PLL LNBF, did you delete all transponders and rescan the satellite? The previously scanned frequencies could be off a few Mhz, This may be enough to throw the 1200's afc to the limit it's capable of locking reliably. With an SR of only 2700 some, it's a pretty skinny signal.
Without rescanning, you may manually shift the TP Freq up or down and find a 'sweet spot'. I'd try 1 Mhz steps to a max of maybe 3 Mhz.
Yeah, I've re-scanned a couple times and even set the receiver back to factory default. I try not to do that if at all possible as I lose the names of the channels I renamed, like the channel in the Virgin Islands and Puerto Rico.