The verbable is not a verb but an adjective.
Thus, the verb preceding the verbable is not an auxiliary verb! It is, in fact, a link verb.
The verbable is usually preceded by the link verbs blijven, vallen or zijn (these are all irregular verbs).
||het blijft te proberen
||it remains possible to try
||het valt te proberen
||it is possible to try
||het was te proberen geweest
||it had been possible to try
The statement that we are not dealing with an auxiliary verb here must be rather confusing as the construction clearly consists of 'te' and a verb. The last example shows us that zijn (in the pluperfect: was geweest) is not an auxiliary verb. Had it been an auxiliary verb for proberen, the past participle would have turned into an infinitive.