DD1 lives several hours away, works full time and spends her spare time driving DGDs to and from after school clubs etc. We see them two or three times a year and between times the only way we stay in contact is via FB and WhatsApp. Comments I put on FB have to be carefully thought out as all her friends see them too, so, in the OPs case a perhaps remark such as "I'll send your birthday pressie as soon as you forward your address, love mum xx" may jog her into a response?
I have found WhatApp to be a boon. Myself, my two DDs, DS, and DiL are in a group. Pictures are shared and comments fly - sometimes a bit embarrassing for DS but he's used to it with two sisters.
Also, if they are luke warm about you visiting, perhaps it's the worry of having you stay? It can be stressful if you've not lived together for a long while. Perhaps they're thinking you'd be shocked about how their home looks, the way they bring up their children, wanting to see them all the time, so many little worries if you've lost touch a bit. If you were to say you're thinking of holidaying in .... and would like to call and see them while you're staying in ... and it would be lovely to take them out for a meal somewhere they wouldn't feel pressured, feel more relaxed and see more of you than planned.