Short answer. Only within a short window. The rule says a defendant waives all defenses and objections not alleged in the answer, but allows the court, in the interest of justice, to permit amendments to the answer not later than ten days from its filing.
What the law says
A defendant waives all defenses and objections not so alleged but the court, in the interest of justice, may permit amendments to the answer to be made not later than ten days from the filing thereof.
Rule 67, Section 3 — Defenses and objections. Read the full provision →
The waiver is the rule; the amendment is the exception
The sentence is built in two halves and the order matters. The default is that a defendant waives all defenses and objections not so alleged — that is, everything not put in the answer. Only then does the rule allow a way back. So an objection left out is not simply pending until someone raises it; on the rule's own wording it is waived unless the exception is invoked. That is a strict regime by the standards of ordinary civil pleading, and it is worth appreciating before assuming there is time.
Ten days, counted from your own filing
The window is short and its starting point is easy to misread. Amendments may be permitted not later than ten days from the filing thereof — that is, from the filing of the answer, not from the discovery of the omission or from any later step in the case. A defendant who realises weeks afterwards that something was left out is looking at a period that has already run. Check the date your answer was actually filed before doing anything else.
Permission is discretionary, not automatic
Even inside the ten days, the rule says the court may permit amendments, and it qualifies that with in the interest of justice. That is a standard the court applies, not a box the defendant ticks. Filing within the period is therefore necessary but not by itself sufficient, and the request is better made with an explanation of what the omitted objection is and why allowing it serves that interest, rather than as a bare application to amend.
What is not lost even if the window has closed
One thing survives the waiver, and it is the issue most owners care about most. The same provision states that at the trial of the issue of just compensation, whether or not a defendant has previously appeared or answered, he may present evidence as to the amount of compensation and may share in the distribution of the award. So a missed objection does not silence you on price. Given how short the period is, take the answer and its filing date to a lawyer immediately.