Can you place your feet inline off ice in shoes or bare foot? I assume so, in which case it’s a matter of balance or lack thereof. Similarly can you stand with your right foot in front of and to the left of your left foot off ice in shoes or bare foot? Again I assume so. If not, then yes flexibility is the issue.
IMO crossovers are a fairly advanced skill. Most people I see - apart from good skaters - can sort of crossover but their feet never actually cross over. It’s hard because you have to lean in an unnatural position that is unstable when stationary. You also need lots of knee bend, lots of leg extension and the crossing over leg has to reach right over. And you need to ride inside and outside edges, and them outside beasties is hard. FWIW I’ve been learning three years, about six hours a week, but only relatively recently has my ACW forward crossover been quite decent, and my CW is getting there. Someone recently asked me why I was so much faster than him but I had slower legs, and its all about deep knee bend, long reach, long leg extension.
Anyway, that reminds me, take it slowly, lift your leg slowly, place it down push and so on. A common mistake is to do lots of fast steps, with little leg stretch, and go too fast. I agree with the others, you need to know your edges. Our coach recently did drills to prepare people for crossovers. Basically what spinZZ said, do a figure of eight on inside edges, then outside edges. It’s hard, frustrating, and annoying, but it’s very important.
That reminds me, years ago at Guildford I saw a little kid skating in hockey skates on a circle on an inside edge on one leg, then flip round to backwards on one leg on an outside edge, then back again and so on. I haven’t seen such good hockey skating before or since.