There can be many reasons why any compound may not retain in any given HPLC Method, however in this case, triphenylamine has three bulky phenyl groups (see structure below), which can create steric hindrance from interaction with the stationary phase due to the tertiary amine.

NOTE: Tertiary amines are the least polar amine and will have less retention than primary or secondary amines in Aqueous Normal Phase ANP HPLC methods.