By the way, please could someone tell me if I can remove the pronoun "I" in every sentence. I feel I'm writing like a child.
English doesn't drop pronouns based on context like other languages do, I'm guessing that your native language would be Spanish. While it might seem tedious to someone that is used to not using them, everything you've written so far (pronouns and all) is completely normal English.
With regards to your problem, how much do you know about the settings in the Trigger menu? Scopes have varying levels of sophistication, but these are the most important ones:
Mode: Auto, Normal, and Single
In Single mode, the scope only stores one waveform the first time it is triggered. You need to manually reset it for it to capture another one. This is a very useful mode to capture infrequent, transient signals and take your time analyzing them.
In Normal mode, the trigger resets automatically after each capture. This is the best mode for viewing a periodic waveform, because it will stay stable on the screen.
Auto is almost the same as normal, but if there is no trigger event without a period of time the scope will trigger anyway and capture a waveform. This can help guarantee that
something is drawn on the screen, even if your trigger settings are screwed up. It can also be used to view things that might be difficult to know what to trigger off of at first, like a DC signal.
For most uses, you'll probably want Normal mode.
Type:
This defines what condition the scope looks for to cause a trigger. More expensive scopes can have very sophisticated triggering options, but every scope should be capable of Rising Edge or Falling Edge triggering. For viewing simple waveforms, make sure your trigger is set to one of those options and not something exotic.
Source:
I suspect that this might be your issue. Most scopes can only be set to trigger off of one channel at a time. You should be able to set it to any one of the input channels or External. Make sure it's set to the channel you're currently using, and not a channel that's currently unconnected.