Perhaps one thing to bear in mind is that string sections are mutliple sound-sources, which is what gives them their "fatness".
No two musicians will be playing exactly the same thing. Consider a section of, say, 4 violins. Each violin line is a different performance, usually by a different musician on a different instrument. Each line will have different timing differences, tonal differences and pitch differences.
The violin family has no frets, and every musician will "hear" how to play a particular note slightly differently. E.g. most will tend away from even-tempered scales towards a more "natural" scale, but exactly how much will vary from player to player, the music and any fixed-pitch references (piano, woodwind, guitar, etc) they are hearing while playing. The pitch differences set up a slight tremolo effect similar to running detuned oscillators against a tuned one, but the amount of detuning constantly changes.
Then add in varying degrees of vibrato, variances in bow attack at the start of notes, bow pressure fluctuating during a note, whether the bow is playing a downstroke or upstroke (downstrokes usually have a more pronounced attack), and you end up with something very hard to synthesise even if you start with good samples. Simply slapping a chorus effect on a single string instrument doesn't come close.
It's the subtle variations that go into each part that make a string section sound like "strings". To do this via MIDI and a sampler requires, I suggest, multiple MIDI tracks, each slightly different, and ideally multiple samplers with each one eq'd slightly differently and maybe with slight differences in the envelopes as well. For some reason many string samples/sounds have long, slow attacks. In the real world, string players can play with a great deal of attack and often do, be they traditional fiddlers, jazzers or orchestral section members.
On the other hand, if the strings sound is really an ambience-setting background pad kind of thing, the amount of MIDI work required to make things reasonably convincing is probably overkill. In which case consider a "string synth" kind of sound with reverb, eq it tightly, push it into the background and see if that works.