If a judge said, "I'm not sure that [something] is dispositive to your
case," what does that mean in plain English?  How could that sentence
be rephrased using other terms?

Howdy chriso_312,

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Let's see if you can get an idea of what "dispositive" means.

From the LawNerds.com, Inc. website.
http://www.lawnerds.com/guide/analysis.html

"Often the courts will say 'no single factor is dispositive,'
meaning that one fact or set of facts won't decide the case."

From the Google cached (Florida) Monroe County Sheriff's
Office web page.
http://216.239.53.104/search?q=cache:GdOASsf7xYwC:www.keysso.net/pio/legalupdate/aug99lu.htm+dispositive+definition+legal

"... but the Supreme Court found a single one dispositive
(meaning that it 'disposed of' the matter - that single
issue by itself required a new trial, so the court did not
need to deal with the other issues.)"

So, dispositive comes from dispose.  Let's define dispose; from
the Lexico Publishing Group, LLC. website, Dictionary.com.
http://dictionary.reference.com/search?q=dispose

v. intr.
To settle or decide a matter.
...
Phrasal Verb:
dispose of
To attend to; settle: disposed of the problem quickly. 
...
To get rid of; throw out. 

So, to rephrase your example using other terms:

"I'm not sure that [something] is enough to settle,
dispose, dismiss or throw out your case."

If you need any clarification, feel free to ask.

Search Strategy:

Google search on keywords: dispositive definition legal
://www.google.com/search?q=dispositive+definition+legal

Google search on keywords: dispose definition
://www.google.com/search?q=dispose+definition

Looking Forward, denco-ga

Thank you very much for the prompt, clear reply!

Thanks for the 4 star rating chriso_312.

The line in my answer:

Let's see if you can get an idea of what "dispositive" means.

Should have read:

Let's see if we can get an idea of what "dispositive" means.

Looking Forward, denco-ga