Diketopiperazine formation may occur after deblocking the second amino acid, especially in case of a C-terminal proline (generally: in case of Nα-alkylated amino acids).  The bulky chlorotrityl group prevents this cyclization resulting in premature cleavage.
Fmoc-Pro-DHPP- (D-1830) represents a more acid-stable alternative to 2-chlorotrityl resin.