Pompey boss Steve Cotterill hinted that Pack could be close to making a loan switch to Whaddon Road as he looks to give the youngster further first-team experience after he enjoyed spells with Wycombe and Dagenham last season.
However, Yates seems unsure on whether to bring the 19-year-old to the club.
He told the Gloucestershire Citizen: "I am not sure about Marlon yet and there has been a bit of a hitch on that one. I speak to Steve all the time and it could still happen, but it won't be at the moment and certainly not this week."
Elsewhere the Town boss has six trialists at the club, including former Gillingham and Northampton forward Gary Mulligan, as he aims to make new additions to his team.
"There could be one in before the end of the week and I am also waiting for a couple of loans. I am happy with where we are and we'll have a look at the young boys in the first couple of games."