Changing the duration of a still

I have a still image that will not allow a longer duration to be set after adding it to the timeline. Three methods attempted:
1. Click and drag-The gradient edge is red instead of green
2. Right click-change duration does nothing
3. Right click on another still that is the duration I need, then selecting the option (conform lock with media pool clip) gives this warning: Error: Unable to replace because the replacement clip does not have sufficient content.

QUESTIONS:
1. All stills are the same size and tif format. Why would one still have a designated duration that can not be changed?
2. Why not just use the words: replace clip from media pool, instead of: "Conform lock with media pool clip" so we don't need to search the web to find out how to do such a simple thing. Curious, not whining.
THANKS

Davinci Rsolve Studio 15.0.OB.030
W10 64bit
Intel Core i7-5820K 3.30GHz
6 core
12 threads
32 GD DDR4 RAM
Nividia Quadro K4000 3GB GDDR5 192-bit
SSD-OS
SSD-video editing
SSD-scratch
SSD & HDD-storage
(8 total Hard Drives)

I have a still image that will not allow a longer duration to be set after adding it to the timeline. Three methods attempted:
1. Click and drag-The gradient edge is red instead of green
2. Right click-change duration does nothing
3. Right click on another still that is the duration I need, then selecting the option (conform lock with media pool clip) gives this warning: Error: Unable to replace because the replacement clip does not have sufficient content.

QUESTIONS:
1. All stills are the same size and tif format. Why would one still have a designated duration that can not be changed?
2. Why not just use the words: replace clip from media pool, instead of: "Conform lock with media pool clip" so we don't need to search the web to find out how to do such a simple thing. Your command is very wordy!
THANKS

Kindly provide us your system specs and Resolve type and version number?

RS 14.3 | MiniMonitor | Samsung U28D590 via Spyder5 Pro | DTV 10.9.7 | Win64 8.1 | ASUS ROG G751JL, Intel i7-4720HQ, 24GB, 1TB HDD, 500GB SSD, GTX965M | Mackie MCU Pro | Softube Console 1 | Shuttle Pro 2

In an attempt to resolve stills duration issue, tif files were exported from Lightroom at a smaller size. After conform lock with media pool clip on each tif in the timeline, one of them that was the correct length of :02 now will not allow replacement of the smaller file size with the "unable to process warning". So I kept that still and moved on to editing.

A graphic created in Illustrator was exported as a PSD for transparency then placed on the timeline. Attempting to render an area for playback (that suddenly became choppy when it wasn't before) I returned 30 minutes later and my text is now invisible. I had many text files. None that were the size I need them to be can be seen. I recreated one to see if it was invisible...the size isn't responding as expected-it's too big and once I get it the size I need, it disappears.

I give up and need to put this project behind me so will located version 14 to re-install hoping this project loads in RS 14. I had only good response from beta 14 so took a chance on 15 but can't complete a project this way. BTW: Windows did an update after the first issue and I hoped that would fix it, but now I have the text issue. I located the OS version. Sorry I did not supply that before. W10 Pro v. 1709. OS build 16299.431