The short answer
A party can give up a contract right through its conduct, for example by repeatedly accepting late payments or deliveries without objection. Waiver can limit later claims for breach.
Where it fits in your case
When the other side argues you accepted its performance or never enforced a deadline.
What to expect
Accepting late performance without objection may waive the deadline for that performance and sometimes future ones.
Anti-waiver clauses help, though conduct can sometimes override them too.
Reinstating terms. Clear written notice that strict compliance will be required going forward usually restores the right.
What we need from you
Records of late performance and every response you sent.
Common misconceptions
“Our contract says time is of the essence.” Conduct can still waive it.
“One late payment accepted waives everything.” Not necessarily, especially with an objection.