The club had negotiated a fee for the Bristol City striker and were planning to begin contract talks on Thursday morning, but Showunmi turned his back on the opportunity after deciding to stay and fight for his place in Gary Johnson's line-up at Ashton Gate.
It is understood that the Nigerian international may have other options available to him when his deal with City expires this summer.
The news came as a blow to Wise who was hoping to hand Showunmi his debut at Luton Town yesterday, but the failure has emphasised the importance of recruiting a suitable striker before the transfer window closes on January 31.