You could always take what patterns you already have and try to flesh them out more. Let's say you have two patterns, one a "verse", the other a "chorus". Copy and paste the verse 8 times, then the chorus 4 times after that. Then go through and mute or or unmute the individual instruments as the verse plays out so that it builds into the chorus.
The transitions between patterns will probably be jarring, so you'll have to add variations to ease the effect. Think of the generic white noise swooshes that you've heard before a huge part kicks in during transitions in other tunes. These sorts of effects aren't necessarily melodic, so they won't really tax your creative juices if you've maybe been feeling unsure of how the song should progress. They're more like tools that you just have to program in. It's a good exercise to learn about some of the abstract possibilities of whatever software you use, and to think in terms of texture as opposed to notes.
You'll also have to have an intro, which can be a variation on the verse with minimal notes, like the difference between a plot outline and actually reading the story. You can construct an ending in the same way, just a final variation on the chorus or something.
You might still end up with a tune that's only a minute and a half long, but even then, you can probably extract some of the extra transitions and effects that you've worked on and tweak them a bit, maybe cut to half time or play around with weird sounds in order to make a bridge section. If that's even necessary; a lot of great songs aren't actually all that long anyway, and don't really contain much more than a few parts. At least classic pop, anyway.
I end up making a lot of one-pattern wonders. If I don't have much of a plan for a song when I'm starting it, it's almost guaranteed to end up just being a pattern long at first. It's not necessarily bad unless you try to wring too much out of a pattern, which I was guilty of a lot when I started doing electronic stuff on an Electribe. I always try to remember that my favorite old techno and Berlin School electronica musicians weren't working with much more than a 16 or 32 step sequencer and some overdubs.
Apart from any of what I said above, I still prefer to have written something already before I begin tracking it. There's something about writing on a guitar or keyboard that allows me to keep the flow going without getting stuck after a bar or two, so maybe try writing on another instrument before turning it into a chiptune.
I'm sure getting stuck immediately after beginning a song has something to do with the fact that not only are we trying to write a tune on the spot, but we're also programming instruments, mixing them, throwing effects around, juggling the roles of not only every musician in a band, but also the roles of the producer, engineer, and arranger, all the while comparing ourselves to others we deem to be much better at all those things than we can ever be. Sometimes you gotta say fuck it, this song is a minute long and it's finished.