Further alleged offences were reported relating to the suspect but "no robust direction" was given to the investigation lasting over six months.
Chief Constable Chris Eyre said it "actually demonstrates positive work".
HM Inspectorate of Constabulary (HMIC) reviewed child protection at the force last August.
HMIC, which independently assesses police forces, said it took more than six months for Nottinghamshire Police to arrest the 21-year-old man regarding his sexual relationship with a 14-year-old girl.
Zoë Billingham, of HMIC, said: "During this period, two further offences relating to the suspect, involving the exchange of indecent images with other teenage children, were reported to other forces.
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"We found examples of poor supervision of investigations leading to unacceptable delays, which can leave opportunities for perpetrators to continue offending."
In response to the finding, Mr Eyre said: "When a victim would not engage with us we did not give up and continued to pursue the offender over a series of weeks until we could build up enough evidence to arrest him."
The watchdog also found that "potential evidence was lost" in the case of a 10-year-old girl who had been groomed online, due to officers not having specialist training and delays.
Mr Eyre said: "There are some cases where it is completely appropriate for non-specialist staff to deal with offences which relate to child protection matters, with the help and advice of specialists.
"All officers have been provided with online training and regular updates about child sexual exploitation."
The initial HMIC inspection in September 2014 revealed a 16-year-old girl in police custody went without food or water for almost two days, prompting the follow-up review.
Ms Billingham said: "The force still has not reduced the number of children being detained unnecessarily in police custody overnight when they should have been transferred to alternative accommodation. This still needs to improve."
Mr Eyre said it was the "responsibility of the local authority to find suitable alternative accommodation" for the children.
The National Society for the Prevention of Cruelty to Children (NSPCC) said the report uncovered "deeply concerning issues that need to be addressed".
"There have been serious delays in dealing with cases that led to potentially vital evidence being lost and a suspect possibly going on to re-offend," a spokesperson said.
"Police need to make sure that they act on recommendations to ensure every child in Nottinghamshire is protected."
A suspected sex offender may have groomed teenagers before being arrested owing to "significant delays" by Nottinghamshire Police, a report found.