First off, I think you misunderstood the 'theory' part of social penetration theory. It's a theory researchers use to describe the development of friendships, instead of a method to follow for making friendships:
Social Penetration Theory explains these differences in communication in relation to the depth of interpersonal relationships. Developed in 1973 by psychologists Irwin Altman and Dalmas Taylor, the theory states that relationships begin and deepen through self-disclosure.
So, be advised that SPT is no fail-safe way of making friends, but rather a theory that theorizes on how friendships are formed. That, in my opinion, doesn't rule out the fact that you can consciously use self-disclosure in an attempt to deepen your relationships with people though.
Now, from your comment:
however the introduction of talking about my feelings on it was out of the blue. (with my friend, we were talking about movies for a little while and then I transitioned from talking about the mediocre CGI or great dialogue of one movie, to another movie and this time I talked about how much of an impact on me it had and what it made me feel)
Combined with the fact that you've just been friends for six months, I think you've been doing the self-disclosure thing 'wrong', and that might have resulted from a misinterpretation of the 'sharing your feelings' part. You just barged full steam ahead, while these things are supposed to take time. From wikipedia:
Reciprocity must be gradual and match the intimacy of the other's disclosures.
Too rapid, too personal disclosure creates an imbalance in a relationship that can be discomfiting.
Now, how much time it will take will differ from person to person, there's some evidence that gender does play a role as well in how often and when people decide to self-disclose.
Self-disclosure can be a very useful skill, but only if employed properly. According to this source self-disclosure is seemingly more of a tool to tell people 'I relate to what you're going through right now', instead of making it about yourself:
Counselors must be very careful when using self-disclosure. Otherwise the session can become more about the counselor than the client, and that does not serve the clients’ needs.
If your friends aren't used to have you sharing your feelings with them, doing so out of the blue might very well throw them off. Your own source mentions this as well:
Is the time, place, and information appropriate and/or relevant? (emphasis mine)
From the first source again, a great example on how to disclose effectively:
For example, if a client is upset over their divorce, the therapist may disclose that they, too, have survived a divorce.
I know this is focused on therapists, but the same goes for friendships. Let's have a little disclosure, because I have seen this happening in real life.
Remember Alice? She is on the autism spectrum. Which means, Alice has problems 'relating' to me. If I was talking about a problem or my feelings, Alice wouldn't disclose, because she couldn't relate. If Alice did any self-disclosure, it was at the most random times. Which, after a while, becomes pretty tiring and upsetting: You're trying to be there for someone when they're going through a rough patch, but once a patch hits you, there's never any reciprocity.
One of the other girls from the same group I mentioned in that question is what I'd call my best friend now. Why? Because if I told her I was struggling with personal stuff, she'd be there for me. She'd be able to disclose she had gone through the same thing, or something similar, and be able to reassure me I'd survive. She'd share her feelings on my problems, and not make it about her feelings on her problems. Of course, the same goes otherwise, if she was telling me she felt like crap because she'd had a bad day, I'd ask her why and share my feelings or insights, but focused on her problems. Over the course of about 8 years, our friendship has become more and more intimate, to the point where she's recently felt comfortable enough to disclose some problems with a sexual relationship to me.
Long story short, it seems like you went full steam ahead and started disclosing your own feelings, while, if you want to reach this kind of intimacy with your friends, it might be better to focus on relating to them and thus getting them to talk about their feelings. Ask them non-invasive questions about themselves, you could start with simple stuff like 'how are you'. If they reciprocitate, you can slowly ramp up the invasiveness of your questions over time, like 'You told me you had a date, how did it go'? You can use them talking about themselves to reveal things about yourself by finding analogies and bringing up something similar in your own life (like the therapist-divorce example above). Make it about them, not you.