Where the 155-character rule comes from
It is a working average, not a limit. Someone measured a batch of results, found the cut usually landed near 155 characters, and the number spread because it is easy to remember. What Google actually does is render the description in a specific font at a specific size inside a container of a specific width, then drop whatever does not fit.
The practical consequence: character count and rendered width disagree constantly. A description of 150 narrow characters fits. A description of 138 characters with several capital M and W does not. If you have ever shipped a snippet that looked correct in your CMS and appeared chopped in search, this is why.
The two widths that matter
Desktop results render the description at roughly 14px in a container around 600px wide. Mobile is narrower — meaningfully narrower, not marginally. A description tuned exactly to the desktop limit will be cut on a phone, and most of your traffic is on a phone.
The safe approach is to aim at the mobile limit, not the desktop one. Put the part that must survive in the first 120 characters and treat everything after that as optional elaboration.
When Google ignores your description entirely
Often. If Google decides a sentence from your page body matches the query better than the description you wrote, it substitutes that sentence. This is normal behaviour and not a signal that anything is wrong.
You cannot force it to use your text. What you can do is remove the reason to substitute: write the opening sentence of the page so that it would work as a snippet on its own, and keep the meta description aligned with what the page actually says. When the two agree, there is nothing to gain by swapping.
What is not worth doing
Do not stuff the description with keyword variants. Descriptions are not a ranking factor; their entire job is to earn the click. A description written for a machine reads like it was written for a machine, and gets skipped.
Do not write one description and reuse it across a template. Duplicate descriptions are not penalised, but they make every page in a set look identical in the results, which costs you the clicks that a specific sentence would have earned.
A working method
Draft the description last, after the page is written. At that point you know what the page actually delivers, and you can say it in one sentence without guessing. Descriptions written before the page exists tend to describe the page you intended rather than the one you shipped.
Then put the sentence into a preview and check where it lands. If the important part is past the mobile cut, move it forward rather than trimming the tail — the end of a description is the cheapest thing to lose.
One more thing worth doing: read the first sentence of the page body next to the description. If they are far apart, Google has a reason to substitute one for the other. Aligning them is the closest thing to control you have over what gets shown.