Someone treats an outcome as guaranteed, and you want to say it might not be.
序 · The move
1Name the conclusion someone expects to follow — the thing you are about to hold back.Is there a foregone conclusion in the air? 不見得 only works against an expectation; with none, it has nothing to push on.
2Put 不見得 in front of that conclusion, in the adverb slot before the verb phrase.It sits where 一定 would sit; 不見得 is the denial of that 一定, not a flat 不.
3Leave the conclusion possible — you deny that it must hold, not that it holds.Could the thing still turn out true? If your sentence shuts that door, you wanted 不是, not 不見得.
不是 denies the fact (it is not so); 不見得 denies the necessity (it is not necessarily so). 不見得 leaves the outcome possible; 不是 rules it out.
不一定
不一定 is the plain everyday 'not necessarily', flat in tone; 不見得 carries a lean against an expectation — the speaker is pushing back on something taken for granted. Same job, 不見得 is the more pointed register.
不見得他會來 → 他不見得會來 (不見得 sits in the adverb slot before the verb, not at the head)
English 'not necessarily' can stand alone as a whole reply; Chinese 不見得 also answers bare, but inside a sentence it must sit in the adverb slot before the verb phrase, not float at the front like the English clause adverb.