Those with the means and resources, especially combined in groups, could and did successfully flee the siege of Antioch (1097/98).

Stephen of Blois(c1045-1102) withdrew from the camp to Alexandretta and from there to the emperor Alexios, just before the final assault on its walls. However, Stephen had a considerable force still available to him - at least in the several hundreds and probably in the thousands, so the immediate risk of ambush and/or kidnap was remote. A lone deserter or even small band would not have had such numerical safety and even Stephen's force was inadequate to fight a path out of the region.

William de Grandmesnil, Bohemond de Taranto's brother-in-law, joined the ranks of the rope-danglers: those deserting the besieged city at night by shimmying down ropes and away from the relief force under Kerbogha. So rampant were rumours of desertion at this stage of the siege that security was needed to keep the Latins in, not just the Muslims out. William joined with Stephen and the group was able to find passage with ships. This would have been crucial in a hostile region where reducing your time spent in the field and on the road could be life-saving. The ships were most likely Italian merchant vessels.

William the Carpenter(10??-110?) and Peter the Hermit(c1050-1115) both attempted to flee from the siege and, as you rightly say, they were chased down. Bohemond had William returned to the crusader camp by his fellow commander and nephew, Tancred, although he was ultimately spared any harsh punishment (William had previously deserted in Spain and would for a third time in the East, also joining Stephen). Likewise, Peter overcame the shame of his flight and was returned to the Latin fold, as is witnessed by his later activities on their behalf. Prominent figures such as these would be sought out and returned after deserting, where the opportunity arose.

Of course we must remember that these events are only recorded precisely because the men involved were people of note and that for the bulk of 'rank and file' deserters buying passage on ships or sheltering with great lords was unlikely to be an option. Islamic culture in the Levant was a slaver culture and untold numbers of Westerners captured during the crusade era where sold into bondage, or else summarily beheaded. Simply returning along the land route from whence they came would not be any less challenging for a crusader either - even with the provisions and guides of the great combined host it was perilous; alone it would be more so.

What of those that did return? The hundreds of crusade deserters (maybe low thousands) that did return to the West without fulfilling their commitment were scorned harshly. Their more successful former comrades, returning from Jerusalem later (mostly after the battle of Ascalon, 1099) could revel in some degree of public adulation, but the deserters endured humiliation. Stephen and many others joined the Crusade of 1101 to regain some measure of prestige and pride. He and most of his comrades were killed a the battle of Ramla (1102).

I hope this goes some way to helping!

For a good start, see:

T. Asbridge, The First Crusade (2005)

S. Runciman, A History of the Crusades, V.1 (1951)

C. Tyerman, God's War (2006)