Been there, done that...
on the DOVER sector a few times.
TMA departing aircraft climbed to FL170 and transferred from terminal control to en-route.
Frequency very busy, aircraft levels at FL170 for say 10 miles.
Bit of a waste of time asking, "are you ln frequency? and confirm you are maintaining FL170"
Issue the instruction to "continue present heading/Fly heading XXX and Climb FLZZZ"
Aircraft reads it back, hunky dory. Aircraft doesn't read it back, then probably not on freq and TC are v.busy too.
OK, it's not ideal but sometimes it has to be done for a few minutes to hold on while someone wanders back from the smokers Hut to split the sector.
Of course if said aircraft was observed bobbing along at FL173 for 10 miles, the above wouldn't be a good course of action!