Q: For years, I've been storing my camera and phone pictures to my online Google Photos account. (I download the camera photos to my PC and upload them to Google Photos from there.) But for the last couple of months I either haven't been able to upload the camera photos from my PC, or could only upload some photos but not others. Meanwhile, photos I take with my phone are still being uploaded to Google Photos. (I don't think this is caused by a shortage of online storage space, because Google Photos is set to "free unlimited storage.") What's wrong?

Rick Schultz, Roseville

A: I think you have two problems: The Google Photos settings on your PC don't match those on your phone, and your unlimited storage actually does have a limit.

Normally, Google Photos must share its 15 gigabytes of free online storage with your e-mail (Gmail) and cloud data storage (Google Drive.) One advantage of this sharing arrangement is that Google stores your photos at their "original" quality — they retain the amount of data they had when you took them.

But, as you've discovered, Google also offers another choice called "high quality," which comes with the promise of "free unlimited storage." Under this option, Google Photos uses a technique called "data compression" that discards some of the data in each photo you store, permanently lowering the photo's visual quality (a 10-megabyte photo will be shrunk to 2 or 3 megabytes.) Because these compressed photos take up less storage space, Google gives you something in return: Your "high quality" photos will no longer count against your 15 gigabytes of shared storage; this essentially gives you unlimited photo storage.

Here's where the settings on your PC and phone come in. They connect to Google Photos in different ways, so you must choose the "high quality" photo storage option on each device. While your PC is using the "high quality" method, I suspect that your phone is not.

As a result, your phone is storing photos online in "original" quality, which uses up a larger and larger portion of your 15 gigabytes of shared storage. If you've been doing this for years, your shared storage may be approaching zero. Once your Google account is full, you won't be able to store any more data — even your "unlimited" photo storage will stop.

Running low on storage could show up in a couple of ways:

• If your 15-gigabyte allotment is nearly used up, you might be able to upload a limited number of additional pictures to Google Photos. This would explain why, on some days, you could upload some photos and not others.

• Your remaining available storage might fluctuate daily as a result of adding or deleting data from Photos, Gmail or Drive. As a result, you might be able to store additional photos one day but not the next. This would explain the times that you couldn't upload photos.

What can you do? Delete some data, or buy additional storage capacity (Google charges $2 a month for 100 gigabytes.)

Alternatively, you can reclaim some storage space by using the Google Photos "recover storage" button (below and to the right of the "high quality" box you've already checked.)

That will downsize your existing "original quality" photos to "high quality" so that they aren't counted against your 15-gigabytes of shared storage. You should also change your phone's Google Photos setting so that, in the future, it stores photos online at "high quality" instead of "original." (To change the Google Photos picture quality on any device, see tinyurl.com/yc3hcuf7).

E-mail tech questions to steve.j.alexander@gmail.com. Include name, city and telephone number.