Never tried this personally, but people tell me, that sonication of grignards is a top class way to initiate ones that are being uppity and refusing to start, using an ultrasonic cleaning bath, apparently it greatly increases tolerance of the reaction to water, allowing, seemingly, even ether from half-used cans that have been exposed to the open air to form the grignard.
THF is preferable to ether, as the exposed oxygen atom has a stabilising effect (I think it forms some sort of intermediate complex, but I'm no expert on grignards), dioxane likewise, but dioxane is of course, toxic, hard to source and pricy as blazes.