Possible problems:
1- mash process produced too many non fermentable sugars resulting in heavier body and less alcohol production. too low temperature, incomplete conversion before mashout. Iodine testing and thermometer placement and degree of accuracy. check with boiling water and calibrate
2. weak, incorrect strain, or old stale yeast. Even with starter, you wont get good results. proof of activity a must.
3. oxygenation of wort prior to pitching inadequate or non existant. Splash it around when cooled, be sure to stir in plenty of oxygen. After it starts fermenting is too late.
4. fermentation temperature not optimal for strain of yeast used. Make sure the strain you use matches your fermentation temperature or results will be poor.make sure you can maintain a stable temperature.
5. grist grind, water to grain ratio; grain freshness, mash ph not optimal, mash profile not suitable for type of brew and / or grains used , insufficient available ions for optimal mash- eg water too soft and not enough minerals present. using spring water helps.
One or a combination of any of these factors will leave your wort hanging at a high gravity. Especially with high percentage of wheat in the grist these factors are critical, and is the reason multiple step decoctions are traditionally used in making wheat beer to address most of these issues.All you can do now is wait it out. most wheat beer yeast require warm temperatures to work, try a heat belt if too cool. As for raising the temperature for diacetyl rest, that can be done when in the bottle after priming. CHEERS!