Could be old, stale 2 cycle gas/oil mix that you put in their tanks?

Some considerations.
First of course the already mentioned fuel quality. 2 strokes tend to be a bit higher in compression ratio and poor fuel is simply going to have trouble firing off a cold engine.

Proceedure. Using purge to assure carb is full of fresh fuel. Correct pulls using choke. You have to listen for that blurb that says it wants to go and take it off of choke immediately or flood it. That blurb may be a pronounced short run to nothing more then a tiny burp. Each engine is a little different and you have to know what yours is saying. If in doubt remove choke after 3 pulls, then give 3 more without before choking again.

2 strokes like throttle when cold. If your unit has no trigger lock you need to hold it while cranking. Half throttle is best but if hard to hold at that point just give it full throttle. Once warm, re-starts usually need no throttle.

2 strokes like sharp crisp pulls when cold. If your physical condition makes this difficult you are in trouble. Over the years I would say the majority of folks in our shop who just can't get their cold 2 strokes running have some sort of physical impairment that makes it difficult for them to provide the required pulling action to easily start 2 strokes. Their mowers have long ropes which they can gather rpms through but they simply can not get that guick crisp pull needed on their little 2 strokes.

Thanks for your replies, guys. As I said in my original post, the trimmer was taken to the dealer last week because it wouldn't start. I had changed the gas to fresh with appropriate amounts of Stabil and Stihl oil additive for 1:50 mix as directed. New air filter. The dealer put in a new spark plug and gave it back. Now it doesn't start again. I used the same pathetic pull on the cord that started it most every week last year. No burble, no blurb.
I'll just try to pull harder.
Norm