手に[入らない]{はいらない} vs 手に[入れない]{いれない}
In this case it's a transitive / intransitive distinction.
The dictionary makes this pretty easy to see:
手に入{はい}る: 自分の所有となる。
手に入{い}れる: 自分の物にする。
See the difference: なる
vs. する
. Pretty much the same as the original verbs: はいる / enter (intransitive)
vs. いれる / enter, put in (transitive)
.
So you could think of it as 手に[入る]{はいる}
/ "enters my hand" and 手に[入れる]{いれる}
/ "put in my hand" (not a proper translation, but good as a mnemonic).
From this, if you want to say "I can't get my hand on it" as you were supposing, you would rather say 手に[入れ]{いれ}られない
(potential form - can't - of the transitive alternative) and this would imply that (even if) you tried actively it wouldn't be possible (as a side note, this returns a lot more search results than なかなか手に[入れ]{いれ}ない
which to me sounds unnatural). When you say なかなか手に[入ら]{はいら}ない
then, it would rather mean that you wouldn't get any (not implying any effort on your side), so as @jlptn1 said "hard to get".
Don't be fooled by the kanji being the same, it is 2 different verbs with related but different meanings.
手に[入らない]{はいらない} vs 手に[入れない]{はいれない}
[入れない]{はいれない} is the potential form (可能形) of [入る]{はいる}. [入る]{はいる} is a non-volitional verb (無意志動詞) here, meaning it doesn't express a voluntary action (as explained in the preceding chapter).
Typically, non-volitional verbs can't be put in potential form. That's why 手に[入れない]{はいれない}
is incorrect.
Note that [入れた]{はいれた}
can be encountered, but as the potential form of the volitional verb (意志動詞) [入る]{はいる}
. For example:
有名大学に[入れない]{はいれない} (I can't get in a renowned university)
家の中に[入れた]{はいれた} (I could get into the house)
That's why the link @user16315 shared mentions [入れる]{はいれる} being restricted to people (and not objects) as subjects: a living being as a subject infers the "entering action" is "voluntary", whereas an inanimate subject infers an "involuntary" one.
手に[入る]{はいる}
is a non-volitional verb, and naturally its subject is an object (切符
), so it can't be put in potential form.